[ES6] Computed properties

ES6 Computed properties of Object Literals.

This diff just adds as a dependency the: https://github.com/DmitrySoshnikov/es6-computed-properties

```js
var x = 'a';
var y = 'b';

var foo = {
    [x + y]: 10
};

console.log(foo); // {ab: 10}
```

Note: main esprima version of the `esnext` project should be updated to address the computed properties feature that recently was merged.