Support JsonSerializable to remove 'resource'.

As shown in https://github.com/ShawnMcCool/laravel-auto-presenter/issues/21, using json_encode() on a presenter wrapped entity results in the internal 'resource' element being visible.

Incorporating support for the JsonSerializable interface (http://uk3.php.net/manual/en/class.jsonserializable.php),  fixes this.

At least for me.

NOTE: PHP V5.4.0+ only - so I don't know how you want to incorporate this into the current release - or if you want to at all.

If there is a way, then that would be great!