DATAJPA-622 - AbstractPersistable.isNew() is now annotated with @Transient.

We now mark AbstractPersistable.isNew() property as @Transient for JPA in order to make sure that no JPA provider complains about a missing setter in case a user derives a domain entity from AbstractPersistable. Without this it isn't possible to bootstrap a PersistenceContext.

Original pull request: #115.
DATAJPA-622 - Polishing.

Renamed CustomAbstractPersistableIntergrationTests to AbstractPersistableIntergrationTests. Reordered declarations in persistence(2).xml to make sure we retain alphabetic sorting.

Original pull request: #115.