protobuf: Clean up and update to version 2.6.1.

The upstream URL has changed as well as the download URL. I'm using the
archive from GitHub directly now, because version 2.6.1 hasn't been
uploaded to Google Code yet.

Changed the meta.license attribute to BSD3, because I haven't actually
found anything regarding "mBSD" on the web, so I'm guessing it should
mean "modified BSD". After digging up a bit more, there seems to be
overall consensus on BSD3/"new BSD" license, because it's BSD3 with
copyright of Google Inc. on the license.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>