Allow the default persistence unit to register entities

[johnament]

Presently, if you want to register entities in the default PU, they need to be discovered, or fall into the default discovery mechanism. Hibernate specifically has package level annotations. To register them, you register the package as a class. There's no easy way to register them, so the idea is to introduce a config property that includes a comma separated list of properties.

Property will be hammock.jpa.__default.entities

FYI: @derekm