(unfinished) updated install docs: Python 3.4, virtualenvs, wheels.

Todo:

* [ ] check that Win 64 install instructions work;
* [ ] add Win 32 install instructions?
* [ ] finish Ubuntu install instaructions;
* [ ] add Debian install instructions?
* [ ] check that everyone is OK with recommending Python 3.4 + virtualenvs

Maybe, it's better to use default python. For 12.04 it could be python3 from apt-get (given that ubuntu provides numpy, scipy and scikit-learn for it) or python2.

For newer ubuntus it should be python 3.

I would suggest using apt-get as much as possible and don't rely on virtualnv (as it requires knowledge about python deployment)