Add javadoc to NinePatch

Explain the difference between "pre-processed" (i.e., ".9.png" files)
and "post-processed" NinePatches.  Add disclaimers on the constructors
that create degenerate patches.

Use `final` in a couple more places, especially in the draw() method.
Use the TOP_LEFT, etc, constants within the basic constructor.

Add a simplistic NinePatch test case.