replace octicons-regular-webfont with official GitHub Octicons

So far we've used the GitHub icons that have been made available through their [Android app](https://github.com/github/android/blob/master/app/assets/octicons-regular-webfont.ttf), but now that http://octicons.github.com is out, this replaces the Android icons with the newly released ones.
Merge branch 'octicons'; fixes #31