Separated UnitOfWork and EntityManager

[jjantunen]

This modification uses work of Andreas Viedma
(andresviedma:unitofwork-annotation).

Current solution of the EntityManager doesn't robustly support use of
Transaction scoped EntityManager.If EntityManager is injected or
emProvider.get() is called outside of the Transactional annotation,
then this will lead to never closed EntityManager. (Current solution
would need boilerplate UnitOfWork.end() calls). This behaviour is also
different than in Guice 2.0 with warp-persist, where Transaction scoped
EntityManager works.

With this modification UnitOfWork and EntityManager are separated.
This means that getting of EntityManager doesn't start UnitOfWork.
If EntityManager scope needs to be different than transaction,
UnitOfWork has to be explicitly started.UnitOfWork can be started
with annotation or explicitly calling begin(). Annotation supports
nested UnitOfWorks. If begin is called directly from UnitOfWork then
it requires direct call to end() to end UnitOfWork.End call will always
end UnitOfWork.

Resolves issues: #739, #947

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