Better installation docs, how to install from source, removing virtualenv tutorial.

Two commits !!
There messages should be self explanatory.

These concerns only the ``installation.rst``.
well, doesn't it concerns also the "install from source" part ? It's a long and incomplete mini-tutorial on virtualenv, a soft that is optional for tryton. I'm afraid we are really bloating things, especially in a administrative guide used to have a practical install guide. Additionally, you are bloating up the experience of the reader with hassles that break the beautifull elegance and simplicity of the sole "pip install tryton" command. Installing tryton might be unadvertently advertised as long and complicated...
I'll put it wherever you want me to put it, but I really don't think it's a good idea to mix things up.