Prevented mocking domain classes when HibernateTestMixin is present on the test

Trying to mock domain classes when HibernateTestMixin was also used on a
test results in an exception. This patch tries to detect
HibernateTestMixin and stops the domain class mocking if it did.
Detecting HibernateTestMixin in superclasses as well, now
Merge pull request #33 from gregopet/master

Plugin causes exceptions when HibernateTestMixin is also used on a test