Simplified property conversion logic in converter.py
Added ObjectMissingException to be raised when GET/PUT/DELETE calls can't find appropriate model instance.
Fixed bug where passed in cursor wasn't used properly (called cursor() instead of with_cursor())

If key is integer ID, return integer ID, else return string key. This makes it so that callers passing in an int will receive an int back, and can use unittest.AssertEquals().
Raise ObjectMissingException instead of ApiFailureException when model instance cannot be found.
Added test_CRUD to test Create/Read/Update/Delete of one model instance.
Added test_delete_all_fruit to test looping through models using cursor and deleting them.