Added PaginatorAwarePass CompilerPass.

This makes it possible to tag services that the paginator should be
injected into. Those services need to implement PaginatorAwareInterface.

- Added PaginatorAwareInterface and PaginatorAware base class.
- Added CompilerPass to scan for 'knp_paginator.injectable' tag
- Added tests for PaginatorAwarePass
- Added Compiler to KnpPaginatorBundle class
- Added documentation about the DIC tag.