Created the PageId.

The `PageId` represents a unique identifier for a story page. The two
member fields have getters and setters and have unit tests written to
check that the setters validate input correctly.
Created the StoryPage.

The `StoryPage` represents a page of a story. No unit tests are needed
yet because the getter and setter for the `id` currently does no
validation.
Deleted ModelTest.

Unit testing for the `Model` is not needed right now.
Upgraded the GAE SDK from 1.8.7 to 1.8.9.

I noticed that I had not completely updated my environment and figured
that because I was early in this process of rebuilding my project, the
upgrade was unlikely to affect my work. Retesting my unit tests and
relaunching the “Hello world” server confirms this.
Sorted the methods of PageIdTest alphabetically.
The StoryPage can now be created in the datastore.

* The build environment needed extra helper JARs added for testing the
datastore interactions.
* As much as possible of the actual creation process was put in the
`Model` so that it can be mostly reused by other `Model` subclasses.
The only elements that were delegated to the `StoryPage` were the ID
related properties, which the `Model` doesn’t know about.
* Partially resolves issue #2.
Updated the Readme file.

I followed some of the advice at
http://stackoverflow.com/questions/2304863/how-to-write-a-good-readme