Added a description to one of the getters in StoryOption.
Implemented StoryBeginning.create().

* Built a new object called `StoryBeginning` to represent a pointer to
the first page in a story. Because it is a subclass of `Model`, I
implemented all of the necessary abstract methods.
* I built a unit test for `StoryBeginning.create()`.
* I added the `StoryBeginningTest` unit tests to the `AllTests` unit
test suite.
* These changes partially resolves issue #4.