Refactor ChunkyPNG::Point factory to improve readability, reduce nesting.

I have split up long lines and semicolon one-liners to improve readability.  I
also pulled some of the logic into a private helper method so that anybody
reading over this can immediately get a high-level understanding that's a little
tricky with deeply nested case statements.
Refactor ChunkyPNG::Dimension factory for readability.
Change simple case statement back to one-liners to match project style.
Merge pull request #65 from jhamon/jhamon-refactoring

Refactor ChunkyPNG::Dimension and ChunkyPNG::Point factories for improved readability.