Automatically detecting smart Git hosts

Added logic to automatically detect smart Git hosts that allow shallow cloning. This is done by sending an `ls-remote` request to the server and then evaluating the returned HTTP header fields. For this, Curl
verbose logging is enabled for the `ls-remote` request, since Curl verbose logging sends the returned HTTP headers to `stderr`.

If the `stderr` output contains the desired header

    Content-Type: application/x-git-upload-pack-advertisement

then the server supports shallow cloning.

This approach uses Git and Curl for the heavy lifting. Instead of implementing the request to the server using a simple HTTP client, Git is used, since it takes care of authentication using stored credentials.

The used approach should also work for BitBucket, which only sends the Content-Type header when a specific user agent is used. Using Git to make the request enables this behavior.

The function to detect the smart Git host (`GitRemoteResolver.prototype._supportsShallowCloning`) returns a promise that is resolved when the server's request is evaluated. The promise handling required an addition to `GitHubResolver.js` - to always resolve the promise to `true`, since GitHub supports shallow cloning.

This should fix the issues #1558, #1559 and #1568 - and provide a better solution than #1393, which disabled shallow cloning for everyone.

I don't have an instance of GitHub Enterprise that I could test this with - but I'm pretty confident that it is working. It would be great if someone with access to a GitHub Enterprise instance could verify this.

If you run Bower with `-V` (for verbose logging), you should see a message indicating whether the host in question (for non-GitHub hosts) supports shallow cloning, i.e. is a  _smart_ host.

The code could be optimized to cache information about the hosts - I haven't added this yet since I wanted to get feedback on whether the approach I've taken is sensible/acceptable.
moot as stderr is already a string