Error handling and cleanup

Added `_bundleError` which gets called when `prepareBundle` encounters an error. Now when encountering an error (only in `developmentMode`) `result.source` gets replaced with a `document.write` of the error. This is rather "in your face" but that's what I would (and do :smile:) want when in developmentMode. Rather than having to restart the process after looking at the thrown stack trace, the error gets presented to me in the browser and I can fix, save the file and refresh the browser and it will work.

*Other minor changes*
- Made the sample app work
- Cleaned up some unused files
- Fixed a few lint errors in index.js