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.
Actually, just read the other comments. It seems like "key" is more appropriate if this just relates to object literals.