refact(es6): use es6 syntax.

* fixed #10, But iojs-2 not support Object.assign, so use object-assign
  shim instead for now.
* use es6 syntax.
* use eslint to instead jshint.

benchmark/flattening.js

@@ -0,0 +1,203 @@
+'use strict';
+
+const Benchmark = require('benchmark');
+const benchmarks = require('beautify-benchmark');
+
+const suite = new Benchmark.Suite();
+
+function isObject(obj) {
+  return Object.prototype.toString.call(obj) === '[object Object]';
+}
+
+function flattening(data) {
+
+  const result = {};
+
+  function deepFlat (data, keys) {
+    Object.keys(data).forEach(function(key) {
+      const value = data[key];
+      const k = keys ? key : keys + '.' + key;
+      if (!isObject(value)) {
+        return result[k] = String(value);
+      }
+      deepFlat(value, k);
+    });
+  }
+
+  deepFlat(data, '');
+
+  return result;
+}
+
+function flattening_1(data) {
+
+  const result = {};
+
+  function deepFlat (data, keys) {
+    Object.keys(data).forEach(function(key) {
+      const value = data[key];
+      const k = keys.concat(key);
+      if (isObject(value)) {
+        deepFlat(value, k);
+      } else {
+        result[k.join('.')] = String(value);
+      }
+    });
+  }
+
+  deepFlat(data, []);
+
+  return result;
+}
+
+function flattening_2(data) {
+
+  const result = {};
+
+  function deepFlat (data, flatKey, key) {
+    const value = data[key];
+    if (isObject(value)) {
+      Object.keys(value).forEach(function(k) {
+        deepFlat(value, flatKey + '.' + k, k);
+      });
+    } else {
+      result[flatKey] = String(value);
+    }
+  }
+
+  Object.keys(data).forEach(function(key) {
+    deepFlat(data, key, key);
+  });
+  return result;
+}
+
+const resource = {
+  'model.user.foo.bar.aa': 'Hello',
+  model: {
+    user: {
+      fields: {
+        name: 'Real Name',
+        age: 'Age',
+        a: {
+          b: {
+            c: {
+              d: {
+                e: {
+                  f: 'fff',
+                },
+              },
+              model: {
+                user: {
+                  fields: {
+                    name: 'Real Name',
+                    age: 'Age',
+                    a: {
+                      b: {
+                        c: {
+                          d: {
+                            e: {
+                              f: 'fff',
+                            },
+                          },
+                        },
+                      },
+                    },
+                  },
+                },
+                post: {
+                  fields: {
+                    title: 'Subject',
+                  },
+                },
+              },
+            },
+          },
+        },
+      },
+      model: {
+        user: {
+          fields: {
+            name: 'Real Name',
+            age: 'Age',
+            a: {
+              b: {
+                c: {
+                  d: {
+                    e: {
+                      f: 'fff',
+                    },
+                  },
+                },
+              },
+            },
+          },
+        },
+        post: {
+          fields: {
+            title: 'Subject',
+          },
+        },
+      },
+    },
+    post: {
+      fields: {
+        title: 'Subject',
+      },
+    },
+    model: {
+      user: {
+        fields: {
+          name: 'Real Name',
+          age: 'Age',
+          a: {
+            b: {
+              c: {
+                d: {
+                  e: {
+                    f: 'fff',
+                  },
+                },
+              },
+            },
+          },
+        },
+      },
+      post: {
+        fields: {
+          title: 'Subject',
+        },
+      },
+    },
+  },
+};
+
+//console.log('flattening:', flattening(resource));
+//console.log('flattening_1:', flattening_1(resource));
+//console.log('flattening_2:', flattening_2(resource));
+
+suite
+
+.add('flattening', function() {
+  flattening(resource);
+})
+.add('flattening_1', function() {
+  flattening_1(resource);
+})
+.add('flattening_2', function() {
+  flattening_2(resource);
+})
+.on('cycle', function(event) {
+  benchmarks.add(event.target);
+})
+.on('complete', function done() {
+  benchmarks.log();
+})
+.run({ async: false });
+
+//$ node benchmark/flattening.js
+//
+//  3 tests completed.
+//
+//  flattening   x 32,863 ops/sec ±0.83% (98 runs sampled)
+//  flattening_1 x 10,434 ops/sec ±0.73% (96 runs sampled)
+//  flattening_2 x 21,734 ops/sec ±1.04% (95 runs sampled)