Reordering steps in setup section of README

Running ./run_tests.sh requires an activated virtualenv
Adding setup text regarding optional virtualenvwrapper

This information was missing and changes when the virtualenv should be activated
Changing numbering of setup steps to match how GitHub renders them in HTML

This makes it consistant with the way it's rendered by GitHub and the other numbered lists in the file.
Merge pull request #195 from alphagov/reordering-setup-in-docs

Reordering setup in docs