Merge @aljosa's bitbucket work; #149

Merge branch 'bitbucket_support' of https://github.com/aljosa/www.gittip.com into bb

Conflicts:
	www/%participant_id/index.html
Add BB key/secret for local dev; #149
Wire up Bitbucket oauth; #149

I tested connecting a Bitbucket account to an existing account in both
merge and non-merge cases. I need to look at signing in using Bitbucket
in the first place, making sure we have links in all the right places.
Bump version to 7.3.0
Bump version to 7.3.0-dev