Reference Aware Behat Contexts

- Added `FixtureServiceAwareContextInterface` that allow contexts to reach the fixtureService
- Added a trait for easy implementation that allows resolving references
- Made use for the ProxyReferenceRepository and its caching habilities to cache references along with DB (recommend Doctrine 2.5+ for bug fixes)
Merge pull request #26 from rdohms/repository-resolver

Reference Aware Behat Contexts