POC - Feature: Repository abstraction for in-memory datastores.

**-- CAUTION --** This is a POC exploring potential of having support for in memory stores.

Basically we provide abstractions for `InMemoryOperations`. The abstraction allows putting any in memory implementation like plain `Map` underneath.  Support for EhCache has been done to have a second implementation to see if it works for more than one type.

So what this currently allows to derive queries for Map implementation using `SpEL` as well as `Criteria` based ones for EhCache. Both implementations are not finished and only support a subset of what's possible.

Have a look at `GenericInMemoryRepositoryUnitTests` as a base and the concrete ones `MapRepositoryUnitTests` and `EhCacheRepositoryUnitTests` to get an impression.

We should try to use compiled SpEL expressions here.
http://docs.spring.io/spring-framework/docs/current/spring-framework-reference/html/expressions.html#expressions-compiler-configuration