PersistentMap backed by JPA

Gives a nice way to retrieve a Map<String,String> backed by JPA (which will eventually be provided by sponge). Since the ConfigAPI and the sponge JPAService are missing, there are some hardcoded bits in JPAProvider and JPAMapFactory. MySQL is only in testDependencies (any other jdbc db should work too).
Due to the missing pieces I'd consider this still a draft.
The name of this interface is kind of misleading: the other persistent types `PersistentMap`, `PersistentSet`, etc all represent a persistent version of the Java interface. `PersistentCollection` is used by both map and set types but does not implement `Collection`. This class should probably have a different name..