The StoryPage can now be deleted from the datastore.

* Added a `delete` method for delete an already existing model in the
datastore. This method was written in the `Model` with no modifications
to the `StoryPage` subclass, so any `Model` subclass that implements
the methods necessary for the `create` and `read` methods will already
have the methods necessary for the `delete` method.
* Created a unit test to test the `delete` method.