Added support for user defined git shorthand

As per twitter/bower/issues/276 here is a pull request to support overriding the shorthand used when resolving git urls.

**.bowerrc**

```
{
    ...
    "shorthand": "git://somehost.com/"
    ...
}
```

Once set rather that using "git://github.com/", "git://somehost.com/" is used as the shorthand.

```base
> bower install jquery/jquery
> bower cloning git://somehost.com/jquery/jquery.git
> bower caching git://somehost.com/jquery/jquery.git
> ......
```