Added :bitbucket as a new :git source

When you are contributed to a gem and have to wait until your pull request gets approved, it is very handy to be able do do:

````
gem 'jekyll-assets', github: 'mcorp/jekyll-assets'
````

I love the `:github` and `:gist` aliases.

I work on several projects where my contractor keeps its source code on [Bitbucket](http://bitbucket.org/) and I'd love to have the same handy aliases we have for `github` also for `bitbucket`.

This patch will make this possible in bundler

````
gem 'flatlab-rails', bitbucket: 'mcorp/flatlab-rails'
````