added callback on start() func
Merge pull request #13 from retasretas/modify-callback

added optional callback on start() function that gets called when the animation finishes.
Callback added and Polyfill added to support IE8

- added optional callback that fires on animation complete thanks to
@retasretas
- added requestAnimationFrame() polyfill to support IE8 and other
browsers without native rAF support
Merge branch 'master' into gh-pages