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.
@nzakas: That rule already exists ([`dot-notation`](https://github.com/eslint/eslint/blob/master/lib/rules/dot-notation.js)). This rule should only be about usage in object literals.