Extract hydration module

This commit will split up the hydrators to a separate branch and fix #33.

Note: this will break BC.
Required changes to the current code:
- Add module to application config: `Phpro\DoctrineHydrationModule` (before `ZF\Apigility\Doctrine\Server`)
- Change config key `zf-rest-doctrine-hydrator` to `doctrine-hydrator` in the generated API configs

Unit tests and travis integration are covered by the external project.