DATAMONGO-348 - Add support for lazy loading of DbRefs.

This is still Proof of Concept!

Introduced DbRefResolver interface in order to be able to abstract how a DbRef is resolved that is used in MappingMongoConverter#doWithAssociations. The present behaviour was to resolve a DbRef eagerly. This functionality is now implemented by EagerDbRefResolver. In order to support lazy loading we have to provide some means to define the desired loading behaviour. This can now be done via the "lazy"-Attribute on @DbRef which defaults to false.
If the attribute is set to true the LazyDbRefResolver is used to create a Proxy that eagerly loads the required data on demand when one of the (non-Object) proxy methods is called. MongoDbFactory now exposes a MongoExceptionTranslator that is now used by the MappingMongoConverter and MongoTemplate.