Avatar Circularization

Rounds avatars in notifications, comments and the reader.

It uses an AsyncTask to do the rounding operation in the background. I think this works ok, but I'd prefer if one of these were possible:

 * Photon could do the rounding for us server-side!!! (I'm trying to track down the Photon gurus)
 * We could update the image cache with the circularized bitmap, so we don't have to apply the circularization.