DATAREST-226 - After release cleanups.

Upgraded to Spring Data snapshot dependencies. Added missing changelog entries.
DATAREST-50 - Added test case demonstrating this issue no longer exists.

Properties with null values properly get rendered in JSON outputs. This was fixed some time ago. Added test case to prove it.

Original pull request: #132.
DATAREST-246 - ValidatingRepositoryEntityListener now uses ObjectFactory<Repository>.

ValidatingRepositoryEntityListener previous referred to a Repositories instance which causes a circular reference on initialization after some changes in Spring Data Commons Repositories.

Major cleanups in the ValidatingRepositoryEntityListener implementation.
DATAREST-238: Change JSON deserializer so it ignores _links

_links are a read-only and should basically be ignored during
any inputs. This required removing the deserializer from
PersistentEntityJackson2Module. A side effect is that any entity
would crash based on the missing _links attribute. Instead of
requiring that every entity apply the necessary annotation,
the mapper is configured to not fail on non-existent attributes.
This allowed all the tests to pass while properly handling PUT
operations.