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 I thought the aim was to have all information at least at one single place. And wasn't the google wiki meant to be replaced by this doc in a undefined point in the future ? If this is the objective we should certainly use the information from their but not link to it ? I'm might be missing some obvious information...