Add sphinx dependency to dev-requirements

[FranciscoCanas]

The section on installing Freeseer for development in the quick-start document (http://freeseer.readthedocs.org/en/latest/quick-start.html) is missing sphinx and python-sphinx for all except the Fedora requirements.

Since the documentation build is part of the test suite, all Freeseer developers should have sphinx installed.

[dideler]

I didn't explain the issue properly, my bad.

Sphinx is intentionally missing from the commands in the Quick Start guide because the dependency is in a requirements.txt file for pip. The problem is that it's in /docs/requirements.txt, while it should be in /requirements.txt since we're now building the docs as part of the test suite.

[dideler]

It's worth noting that one of the students had trouble installing Sphinx using pip, and had to use easy_install. I think this was on Windows. Should investigate the best way to install Sphinx.