Support for "null_value" field attribute. Closes #98
Fix test environment
Fixed tests
Travis: build under 5.5 and 5.6
Travis: better composer install
Travis: use phpunit installed by composer
Merge pull request #106 from fprochazka/patch-4

Fixed travis and existing tests
Fixed consistency

- added objectManager accessors instead of entityManager accessors
- entityManager accessors are marked as deprecated
Implemented DependentMappingDriver