Added Github doclet.

I think that the github doclet is ready to be used: 

### Features ###

* Generates the documentation in markdown format so it can be read from the github source code browser.

* The generated documentation has fully functional links, allowing the navigation through the docs repository.

* The paths are generated with the same case of the namespaces to avoid confusion as the main navigation system is the one given by github.

* It doesn't generate source code files, instead, it allows referencing to a github repository so the original source code can be read.

### Known issues ###

* The links generated from @see tags are broken, as I haven't figured out how to override the original behavior.

* It needs some navigational and esthetic improvements.

## Samples ##

* You can see a sample configuration file here: https://github.com/JeyDotC/phpdoctor/blob/master/examples/github.ini#L25

* And a sample documentation repository here: https://github.com/JeyDotC/Hirudo-docs

**Notes**: Please ignore changes to the standard doclet and other original phpDoctor files, such changes are just code formatting.