Add status receiver for GitHub

Problem description
===================

GitHub provides a simple API for publishing build status for a revision from a repo.

It would be nice is Buildbot could work with GitHub to publish status for builders.

Here is my try.

Please let me know if this is on the right track and what needs to be changed.

This tried to do the same think as: #516


Use cases for the plugin
===================

* [x] Simple projects with a builder using a single repo on internal network and mirrored to GitHub. Define properties on builder and use Interpolation to resolve GitHub repo owner and name. Revision is defined by 'revision' Property. 

* [x] Project with multiple builders for the same repo (multiple repos) but only a single builder (selected builders) will send status to GitHub. Ex same revision tested on Linux and Windows. Define properties on builder and return empty string to skip status sending.

* [x] Simple projects with a builder using a single repo which is hosted in GitHub. Repository owner and name is obtained from Git repo. Revision is defined by 'revision' Property. 

* [ ] Project with a builder using multiple repos but which should send status only to a single github repo. Define properties on builder and use gitHubRepo Interpolation.




Changes requested during review
===========================

* [x] Rename to github.py and force absolute imports
* [x] Use txgithub for non blocking calls
* [x] Move documentation in main documentation and not in the source code
* [x] Add an user defined configuration named 'gitHubRepo' to GitHubStatus. This allows using an Interpolate to specify the name of GitHub repo where to send status.
* [x] Find or add a little tool which can generate a token based on username, password, application keys. This can be an external project and just link to the code.
* [x] Add exception handling
* [x] Add tests


Changes description
===================

This depend on a patch made to txgitub to support github changes. Hope the patch will be accepted soon.

It uses a token for authentication and I have documented how one can obtain a token.

No testes are present. If everything is ok I will to write the tests.

It depends on an external txgithub package but I tried to avoid reinventing the wheel.


How to try and test the changes
===============================

Get txgithub version from here https://github.com/tomprince/txgithub/issues/1:

Check documentation for instruction.

Watch a pull request in GitHub.