DATAJPA-527 - Improved handling of CrudRepository.exists(ID) entities with complex composite id.

We now delegate the exists(…) check to findOne(…) in SimpleJpaRepository for entities that have a complex composite id via @IdClass. Previously we tried to generate a string based count query in that case, which didn't work since the parameter types for the IdClass attributes didn't match the values returned by entityInformation.getCompositeIdAttributeValue(…).

Polished JavaDoc in JpaEntityInformation.

Original pull request: #95.