Added support for Bitbucket along with a custom css skin for it. 

Screenshot added to depict shortcode changes.

I hope you will find this a worthy addition to the plugin! Great job on the GitHub skin, always wanted something like this for Bitbucket on Wordpress.

Signed-off-by: Caspar Cedro <dev@eclecticdimensions.com>

![githubwidgetchanges bitbucket screenshot](https://cloud.githubusercontent.com/assets/8496424/3984190/f1dca246-2885-11e4-999a-6c8509266ad6.png)