Restructure main `janrain.scss` file and imports

A few structural changes that I think make it easier to work with the styles:

- Removed the `lib/` directory which contained a mix of files from
  Bootstrap with old hacks. Moved all of the files in it which were
  actually different from Bootstrap base to an `overrides/` directory
  and discarded the rest.

- Changed the way `janrain.scss` is structured, so that it explicitly
  lists the files its pulling in from `bootstrap-sass-official`. I like
  this better because this way its easier in a subproject to comment out
  the modules you don't need, and get a smaller file size.

Also, remove autoprefixer package & task:

- My default Grunt task still included autoprefixer which isn't needed.
  Removed that. Added jekyll to the default task so that running `grunt`
  will build docs pages as well as compile css.
Does this work now that `variables` has been renamed `_variables`?