Disallow reserved words as keys (fixes #1144)

This PR adds a rule for disallowing reserved words being used as keys, both on object initialisation and assignment:

```js
var superman = {
    class: 'Superhero',
    private: 'Clark Kent'
};

superman['default'] = 'Is it a bird? ...';
```

I suggest it is off by default, as many projects use reserved words as keys. However IE7 and IE8 do not allow this.
@michaelficarra Would you mind helping me out with the docs for this? I feel as if I might word this wrong since I don't have full understanding of the ES spec. Or is it as simple as ES3 not allowing a `ReservedWord` as an `IdentifierName` while ES5 does? Or am I using the terms wrong?