Updating debian dependencies, reaching a state where sbuild works.

* Add dh-python as Build-Dependency, as needed for pybuild.
* Populate python-* package dependencies taken from requirements.txt
* Remove (maybe?) useless python-dev
Create doc/

This directory is going to host the markdown to solve (at least #144).
* Setup a sphinx doc/ directory
* Make sphinx load metadata from the actual globaleaks package.