Support auto type registration during hbm2ddl

As discussed at https://community.jboss.org/message/753496.

One can't make use of Hibernate 4 auto type registration if you also want to run hbm2ddl at some point in the build process. hbm2ddl needs a hook that triggers auto type registration similar to what happens at SessionFactory creation time. Of course the actual user types need to be in the classpath while hbm2ddl runs.