This introduces a BC breaking feature that allows the use of different
pagination systems by means of having a PaginationInterface and
pagination adapters.

    - Adds a PaginationInterface
    - Changes all instances of Illuminate\Pagination\Paginator to be
      League\Fractal\Pagination\PaginationInterface
    - Adds an IlluminatePaginationAdapter to allow use of Laravel's
      illuminate/pagination package out of the box
    - Updates prior tests to change from Illuminate\Pagination\Paginator
      to League\Fractal\Pagination\PaginationInterface
    - Adds a test for the IlluminatePaginationAdapter
    - Updates the README to reflect the changes outlined, and bumps up
      the version to 0.6.0