Readme fixes

Hi Tor team,

In preparation for use on Measurementlab.net, I made some changes to the installation instructions in the readme. I took out references to outdated code based on feedback from art@torproject.org, and made fixes and readability changes based on my own testing. Note that everything below "Using Ooniprobe" is mostly untouched.

I'll look out for any comments or corrections and happily incorporate them.  Thanks!
I'm a little unclear on what's needed, as it's a pretty long script. Here's the apt-related stuff I found, is this more or less what you mean?

Apt sources, in /etc/apt/sources.list.d/* or /etc/apt/sources.list, appended to the file or added with add-apt-repository, where $DISTRO_VERSION is found with lsb_release -cs:

deb http://deb.torproject.org/torproject.org $DISTRO_VERSION main
deb-src http://deb.torproject.org/torproject.org $DISTRO_VERSION main
for DISTRO_VERSION lucid, deb http://archive.ubuntu.com/ubuntu $DISTRO_VERSION universe

Prerequisites:
curl 
git-core 
python 
python-dev 
python-setuptools 
build-essential 
libdumbnet1 
python-dumbnet 
python-libpcap 
tor 
tor-geoipdb 
libgeoip-dev 
libpcap0.8-dev 
libssl-dev 
libffi-dev
python-virtualenv
virtualenvwrapper