DATAREST-95 - Add support for PATCH to partially update entities.

Added general support for HTTP PATCH to partially update entities and amend property resources.
DATAREST-95 - General overhaul of HTTP method handling.

Code polishing in DomainObjectMerger and related test cases. Fixed the related test cases. Cleanups in ControllerUtils to remove unneeded constants and make sure we really render no content for empty responses.

Refactorings in controller classes to reduce code duplication. We now do not allow POST requests for partial updates to property reference resources anymore but require the usage of PATCH.

Tweaked test helper methods to correctly implement basic interaction patterns.

Related pull request: #127.