Add more documentation

And fix some minor issues, mostly labels in AWS and a small bug in CWS.
Improve documentation

Change some sentences that were not clear enough, fix some markup,
correct some spelling mistakes and, most importantly, do not hardcode
the version in the ReST files but use Sphinx's substitution feature.
Since the version was also in the download URL and since no substitution
is allowed there we had to write an extension to do that. We also took
this opportunity to add a new role to more easily reference GitHub
issues. It's largely inspired by the extlinks extension (since it
basically does the same thing).
Fix dependecy lists

Add a few missing dependencies, use their "official" names (i.e. the one
they use on their website), add links to their homepages, update Ubuntu
installation command, add Arch Linux installation command. And do all of
this for both the list in the README and for the list in the
documentation.
Update shebang to explictly use Python 2
Remove debug print
Add a better help comment the the sample cms.conf

Since Arch Linux and Gentoo use, respectively, python2 and python2.7
and both of them install the executable scripts in /usr/bin.
Fix URLs in the README

Fix commit f5e563300a15d95a8c0973e72b6585022e1cba90