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)

The only difference between `get_repo_github` and `get_repo_bitbucket` is how the variable `$url` is defined, why not reuse?

```php
function get_repo($host, $path) {
  switch($host) {
    case "github":
      $url = "https://api.github.com/repos/" . $path;
      break;
    case "bitbucket":
      $url = "https://bitbucket.org/api/2.0/repositories/" . $path;
      break;
  }
  $ch = curl_init($url);
  // tl;dr
}
```