Add Bitbucket buttons alongside GitHub buttons

[SamWhited]

I'd like to see Bitbucket analogs for the GitHub start buttons / banner options.

The following look pretty good: http://bb-btns.bitbucket.org/ FontAwesome has a Bitbucket icon that might be good to use too.

[bitprophet]

+1 on the idea, though I don't have time to do this myself right now.

[buhman]

I opened bb-btns#1, and also re-created it on github pages just to see how it works.

I think it would be better for bb-btns to be fixed upstream first, so that way it's not my fault if I break it. The changes are rather trivial though.