Make MIT license explict

Hi, I see you followed up on issue #8 and stated that a license in `bower.json`. Please merge this PR that makes that choice explicit in the README file and in the code itself.

And please consider [CC0](http://creativecommons.org/publicdomain/zero/1.0/) for next projects, as it avoids the need to have a license file shipped around with each copy of `wallop-slider.js`. ;)