The exploration model should not know who the current user is.
Move add_state() and delete() back to the model class. Add an add_editor() method to exploration.services.
Change states to state_ids for clarity.
Add a ValidationError class to all models.
Rename ValidationError to ModelValidationError so as not to conflict with the Django term.
Add clarifying docstrings.
Merge https://code.google.com/p/oppia

Conflicts:
	apps/exploration/models.py
	apps/exploration/tests.py
	apps/state/tests.py
Remove key from delete method
Add pre-put validation for image format; rename raw property to src_file
Port the changes introduced due to the merge in exploration and state