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/ends-with.js

@@ -1,7 +1,9 @@
-var Benchmark = require('benchmark');
-var benchmarks = require('beautify-benchmark');
+'use strict';
 
-var suite = new Benchmark.Suite();
+const Benchmark = require('benchmark');
+const benchmarks = require('beautify-benchmark');
+
+const suite = new Benchmark.Suite();
 
 function endsWith(str) {
   return str.endsWith('.properties');