Modified the keys on ModelFactory to use the model app

Using Django's ContentType, the get_key value is now generated using the
model's app concatenated with the models name
ModelFactory: Now get_key joins app name and model name with a dot
MockupFactory: model names (in lowercase) can be used as keys again

The name of the model is now also accepted as a key to get a mockup
class. If a second model with the same name is registered then the
model name will no longer work as a key and will throw a
MultipleMockupsReturned exception

This was done for backwards compatibility, and to spare the user to
write the name of the app if it's not necessary
ModelFactory: Fixed a bug when registering a model twice

* If a model was registered twice, the system would believe there was
a model name collision and through a MultipleMockupsReturned if the
model name was used as a key
ModelFactory: On collision, stores a list of mockups Instead of 'None'

When 2 models with the same name collide, now instead of storing a
'None' where in the colliding key, it stroes a the list of colliding
mockups
Tests: New test case for models with the same name in different apps

* Created the zombie_blog app, with models like the blog app:
    Entry
    Comment

* The new tests now checks the default case, before registering the
Entry model from the zombie_blog app. After registration, it checks that
the 'entry' key is no longer valid and raises a MultipleMockupsReturned
Tests: New tests to validate inheritance

* Test to validate that models inheriting from one to another are
correctly creating both the object and the parent model object

* Test to check if __get_item__ correctly throws exceptions when a model
inherits from another with the same name
Merge branch 'master' of https://github.com/juanique/django-chocolate into feature/colliding_model_names

Conflicts:
	test_project/blog/tests.py
Merge pull request #12 from thagat/feature/colliding_model_names

Feature/colliding model names