Rst plugins as classes

* Implemented ReST extensions as docutils Directive subclasses.
* Added test cases for the new implementation, and for the existing extensions.
* Removed code-block in favor of docutils code directive.
* Reimplemented listing directive on top of docutils CodeBlock.
* Updated manual.txt
* Minor fix (removed extra '/' in gist directive causing requests.get() to return empty text body)