Add ability to generate Entity Instance classes with Persistence plugin

The idea of the Entity Instance is pretty much a bunch of boiler plate code necessary to allow a JPA entity to be used by CDI and exposed to EL. See https://github.com/seam/seam-example-confbuzz/blob/develop/src/main/java/seam/example/confbuzz/ConferenceInstance.java for the basic boiler plate code.