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)

1. The structure here is very similar to github's, except for the avatar and bitbucket badge. It should be reused as well.
2. Inline style is a bad idea, should move to CSS file. Since here, the changes made to `wp-reposidget.css` should also reuse rather than copy-paste with only a few differences.
3. (This is my personal view) The avatar is a meaningful part within this scenario. It should be presented with an `IMG` element. CSS `background-image` should be used for visual decoration.