Created tool/hop_runner.dart

Thank you @a14n for creating this library!  I created tool/hop_runner.dart to create Dart Documentation.  Forgive me that I am a novice at github but I wasn't sure how to do a pull request of my gh-pages branch to your library, but you can see the Dart Documentation I generated using the [Hop](http://pub.dartlang.org/packages/hop) library here:

http://damondouglas.github.io/dart-google-drive-realtime/

I'm working on an example using [dart-google-drive-realtime](https://github.com/a14n/dart-google-drive-realtime) and [google_drive_v2_api](http://pub.dartlang.org/packages/google_drive_v2_api)