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

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.

Introduced a DelegatingDbRefResolver that can delegate to a DbRefResolveCallback in order  to perform the actual DbRef resolution.
We now use cglib-proxies if necessary if the referenced association is a concrete class.

Added unit tests for lazy loading of interface types, concrete collection types and concrete domain types. Exposed state from LazyLoadingInterceptor for better testability.

Added unit tests for lazy loading of classes with custom PersistenceConstructor.
Moved integration tests for PersonRepository into its own test class.
DATAMONGO-348 - Polishing of lazy loading implementation.

Extracted DelegatingDbRefResolver and associates from MappingMongoConverter. Let MongoDbFactory expose PersistenceExceptionTranslator only to prevent invalid dependency to core package. Renamed DbRefResolveCallback to ResolverCallback. Removed AbstractDbRefResolver and moved the functionality implemented there (triggering of exception translation) into the DbRefResolver.

MappingMongoConverter now uses a slightly extended version of DbRefResolver so that we can essentially replace the MongoDbFactory dependency with the DbRefResolver one.

Added support for Objenesis based lazy-loading proxies to support domain classes without a default constructor. Explicitly check for Spring 4 being present as with it the default ProxyFactory already supports that out of the box.

Added missing JavaDoc and assertions. A lot of cleanups and removal of deprecation warnings in test cases.