Start implementing Git Data API

Thank you for the great work you did with the `Github.dart` library! I started playing around and thought I'd like to contribute to the Git Data API (see Issue #7).


## Things that were added

* Unittests, enhance the Dart `mock` package so it also works with named parameters.
* Set up a structure for integration tests that work against the real GitHub API.
* More model classes for Git Data API.
* Service for Git Data API (we might want to discuss this part): I've created a separate class called `GitService` that holds the API methods related to the Git Data API. This is just a suggestion. The reason for this class was that I found the `Github` class was getting too crowded with API methods and I think it might be a good reason to divide them up into individual classes (one Service per API group). I took some inspiration from the Java and Go GitHub client libraries for this:
  * https://github.com/eclipse/egit-github/
  * https://github.com/google/go-github/


## Other notes

* I've tried to apply the new [Dart Style Guide](https://www.dartlang.org/articles/style-guide/#comments) recommendation that prefers `///` instead of `/**` for comments.
* There is an [issue](https://code.google.com/p/dart/issues/detail?id=17085) with `DateTime.timeZoneName` so that it currently can't be used. I've removed the `Time-Zone` header.


If you think this is helpful I would be glad to contribute additional parts of the Data API.