Added Enum features for default in DBContext and Class generation. (#243)

* #240
Moved IsManyToManyJoinEntityType extension class. Use IsManyToManyJoinEntityType to prevent InverseProperty and ForeignKey Annotations from being generated in Virtual Tables.

* Added functionality for Enum Defaults and Enums in ManyToMany virtual tables.
#242

Author: mwmccallum