Fixing typos, markup.
Merge pull request #45 from vaab/patch-2

Fixing typos, markup.
updated ``Installation Procedure`` section to add several information.

- a quick description of each part just to know what to install
  - state that modules are addons for the server (which might not be clear to reader)
  - imply that tryton is a client and other client exists.
- removed the ``$`` because we want people to be able to cut and paste
- remaining typo: modules where incorrectly tryton_module instead of trytond_module...
- don't force virtual env, but recommend it.
- where to get list of modules.
- separate clearly each install commands.
Merge pull request #46 from vaab/patch-1

updated ``Installation Procedure`` section to add several information.
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
Refactor installation documentation

* Add a quick install guide
* Re-organize the installation guide for distributions, official releases
  and development setup from source

The documentation layout, content and steps are havily inspired
by the django documentation