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.
Missing license header, execute `./gradlew licenseFormat` (or `gradlew.bat licenseFormat` on Windows) to add them.