Extend taggable-mixin to support different Tag models.
Usage is pretty simple. Tag model is default in Taggable constructor but you
can call it with different model like GHOPTaskType that inherits from Tag model.
Both Taggable and Tag models have been updated and they don't use hardcoded Tag
model anymore and instead use cls of class methods or self.__class__. In case
of Taggable it's self.__tag_model.
A list of bugs is available in the Google App Engine SDK project on Google Code.
The issue tracker is at http://code.google.com/p/googleappengine/issues/.