Entity manager config with connection properties

Added EntityManagerConfig as a way to provide a single instance to the entity manager to save resources and hopefully boost performance.

Included is a possibility to override the connection properties found in resources/META-INF/persistence.xml in order to connect to another database than the default. I have inluded the file (hibernate.properties) with some default values. If desireable in the future, we can add this to .gitignore and provide individual values.