DATACMNS-483 - Support for wrapper types as return types for repository methods.

The repository method invocation mechanism now post-processes the result of the actual method invocation in case it's not type-compatible with the return type of the invoked method and invokes a conversion if applicable. This allows us to register custom converters for well-known wrapper types. We currently support Optional from both Google Guava and JDK 8.

Implementation notice

We use some JDK 8 type stubs from the Spring Data Build project to be able to compile using JDKs < 8. This is necessary as we need to remain compatible with Spring 3.2.x for the Dijkstra release train and our test executions still run into some issues with it a Java 8 runtime (mostly ASM related). These issues were reported in [0, 1] and we'll probably get off the hack for Dijkstra GA by upgrading to Spring 3.2.9.

[0] https://jira.spring.io/browse/SPR-11719
[1] https://jira.spring.io/browse/SPR-11718