Set isRunning to false when tween ends.

- Retabbed src/tween/Tween.js for consistency with other files.
- Set isRunning to false when non-repeating tween ends.

This pull request resolves #221. If you view the first commit with
whitespace comparison disbled, you can see that the only changes were to
he whitespace. The only real change was a single line addition in the
second commit.