DATAJPA-611 - Allow composite keys to be used in Repository.findAll(Iterable<ID>).

We now support an Iterable of composite primary keys to be passed to findAll(..).
Note that since there is no direct support in JPA to perform this query we have to execute a entityManager.find(...) query for every given id, which could lead to performance problems - use with care!

Original pull request: #127.