Some updates to the virtualenv install instructions

* Update some paths
* Will need mysql-dev to build mysql-python
* Workaround for migrations not working from scratch
* Allow people to keep their virtualenvs whereever they usually do.
  (Using $VIRTUAL_ENV in the commands is a little wordier, but people
  will probably be copying and pasting the commands from this page
  anyway.)
* BrowserID is now Persona.
* The shell command `source` is non-standard, and some very
  standards-compliant shells don't have it. `.` should work anywhere.
Merge pull request #663 from dpoirier/doc-tweaks

Some updates to the virtualenv install instructions