DATAREST-248 - Significant overhaul in entity serialization and deserialization.

Refactored PersistentEntityJackson2Module to move a lot of the customization logic into Bean(De)SerializerModifiers. Those allow to modify the Jackson metadata for a given type programmatically so that we can register the appropriate (de)serializers for associations. 

On the serializing side of things this allows us to easily turn associations into links and simply delegate to object serialization as the delegation will simply ignore the association properties as they have been dropped using the modifier. This solves the advanced requirements of DATAREST-117 nicely as all non-association properties are serialized using standard Jackson means so that all customizations apply.

On the deserialization side, we now support URIs as values for association properties to be able to submit references for non-optional associations on creation.

Related issue: DATAREST-117.