Split up MongoDB example into two.

Split up the previously existing MongoDB example project in one for basic stuff, geo-spatial and Querydsl support as well as one on the aggregation framework. This will allow us to add other modules on particular focus areas going forward.
Added multi-store example to Spring Data REST examples.

This app demonstrates how to mix Spring Data JPA and Spring Data MongoDB together (with Spring Data REST on top) to demonstrate how to split them up properly.
Updated readme with latest additions.