Add get_pending_fields method to UserTrackable

In order to avoid reflection in the Auditable class in the form of hasattr(self, 'get_pending_fields'). This field is now implemented in the base class as a no op.

A follow up that further decouples Auditable from Authorizable is in order.
Make plot unicode method fail gracefully

plot unicode methods will now show incomplete information instead of
raising an exception when referenced fields are null.
Expand audit unicode method
Stop tracking instance and change type to a set
Factor out model_hasattr
Replace call to full_clean() with clean()

full_clean() does too much built-in django validation to be
compatible with the changes made in the audit system.
Add new-object creation support for pending edits

Fixes #82 on github.
Phase out god_user from tests

god_user is no longer necessary now that the the audit system
does not track instances and uses ids to determine create
privileges.
Catch exceptions to support apply_change instead of setattr