Add GitHub to social icon set

Hello,

I was playing around a bit with Polymer, and I felt like it was missing a GitHub icon, so I created one.  This should fit in with the various post-* icons.  I used GitHub's `blacktocats.ai` as a source for the octocat image and made it the icon resemble the `favicon.ico` on GitHub's page.

I don't know if anyone else would be interested in it, but just in case, I created this pull request.

Sincerely,

Daniel