More content about how to install from source

- actually I had difficulties on the real topic of the deleted section
- replaced it by a longer version
- mentionned system packages
- and added the complete "python setup.py install" command
- mentionned the github repositories
- added way to browse the official modules repository.
- Added mention to virtual env in both place where we talk about installing python apps.
Merge branch 'vaab-patch-3' fixes #47