Added automatic boolean conversion when setting or fetching model attributes

This pull request allows the user to specific automatic boolean conversion of specific fields on models, much like the existing date conversion functionality. You can do this by setting the $booleans variabled, or by overriding the getBooleans() function and returning an array.

The follow example explains the usage:

```
class TestModel extends \Eloquent {
    protected $booleans = array('completed', 'invited');
}
```

Now whenever you fetch the completed or invited value, you will get a boolean false or true value instead of the normal "0" or "1" strings.