Add support for appends and attribute accessors

[radmen]

Quite often, it's required only to add few more fields to the array created from the model. Instead of overwriting the toArray() method, JsonResource should copy the same pattern used in Eloquent Models.

JsonResource should support the $appends array and use it to call methods following the accessor pattern (i.e., full_name append, should call the getFullNameAttribute() method).

Depends on #1