Divide into service classes (large refactoring)

* Divide the previously huge `GitHub` class up into several service classes. API service calls now have
  the same structure as the GitHub API documentation.
* Remove reference to `GitHub` class inside model classes. Model classes are now just models and
  contain no functionality to call further API methods. Yes, it would be nice to navigate around models but
  this leads to a lot of redundant methods and because it was very different from the API documentation
  it was hard to find the needed methods.
* Added links to GitHub API documentation for every API method.
* Added TODOs for the missing API methods.
Unify naming of API methods
Adjust examples and experiments to new API service methods
Merge pull request #31 from marcojakob/service-classes-issue-27

Divide into service classes - issue #27