Refactor and complete GitHub API implementation

This will refactor and complete the GitHub API implementation for the Joomla! Platform.

This is a "first scratch" - yet a complete one (I hope).
I opened it to obtain some opinions and maybe some assistance - kind of a RFC.

### B/C Issues

Due to the refactoring, some methods have been moved. Since the implementation is still young, I hope this is acceptable.

* [ ] Indetify possibly issues.
* [ ] Provide legacy replacements

### @TODO List

* [ ] Doc blocks in unit tests. (HELP pls ! :) )

### Plans ?

* [ ] Build classes that loosely reflect what is available at [developer.github.com/lib/resources.rb](https://github.com/github/developer.github.com/blob/master/lib/resources.rb)
    These classes could be used by the methods to parse the obtained JSON response, pass it to a specific class and return that class to the caller instead of the standard json_decode'd object.

    Very usefull for auto complete in IDEs. Of course, this should be automated ;)