Allow audio file to be passed as a single string.

- Retabbed `src/loader/Loader.js` for consistency with other files.
- Added a type check on `urls` when getting audio urls.

This pull request resolves #222. If you view [the first commit](https://github.com/kevinthompson/phaser/commit/cdb071d3565eac63acc6ee5e35b548605d4cb1da) with [whitespace comparison disbled](https://github.com/kevinthompson/phaser/commit/cdb071d3565eac63acc6ee5e35b548605d4cb1da?w=1), you can see that the only changes were to he whitespace. The only real change was a single line addition in [the second commit](https://github.com/kevinthompson/phaser/commit/bc4f9da5fa48a5ddea90dc51096bef685bd27315).