'required' child tests

Add tests for 'required' argument to 'MonoClassChild' descriptor.

Add testing models for 'required' child tests.
'RequiredChildError'

Add 'RequiredChildError' exception class.
'MonoClassChild' 'required' parameter

Implement 'MonoClassChild' 'required' parameter. If this argument is
'True', then an exception will be raised before commit if no child is
assigned to a specific parent.
'parent' cascade tests

Add test to confirm that 'parent' object cascades (is added to session)
in various situations.
'after_attach' cascades

Alter 'MultiClass' 'after_attach' listener to add both children and
parents to session on 'after_attach' event.
'propogate_session_add'

Add 'MultiClassRelationship.propagate_session_add' method. If either of
two instances are in the session, this method adds the other.
Merge branch 'dev/child-required' into develop
Prepare release 0.1.1

Update version number.

Update changelog.
Merge branch 'release-0.1.1'