Possible fix for netdna/bootstrap-cdn#115

This pull request is in response to #115.

I essentially copied `glyphicons-halflings-white.png` and `glyphicons-halflings.png` from `bootswatch/2.3.0/img` to `bootswatch/2.3.1/img`. I didn't copy `bootswatch/2.3.0/bootstrap.min.css` because I wasn't sure what that was.