[DDC-497][DDC-500][DDC-342] Fixed. Persister polishing and refactoring. More to come for that, but after beta1.

Changed privates to protected and updated setUp to use late static binding. This allows projects using Doctrine2 to use its test cases for testing their own entities.

[2.0] Added support to IdentificationVariable that was missing in ArithmeticPrimary (it was not correctly handling it). Uncommented a unit test that added coverage to it.

[2.0] Fixed E_NOTICE being throwing when ->getSingleScalarResult() is called and no result is found. Added coverage for this and also for multiple result (NonUniqueResultException).