Update documentation.

Update "Satellite Installation" in readme:

* Update the descriptions of each code example.
* Change the look of the code samples themselves. Use a "sh" code fence to mark
  off each code sample. This'll make the sample look nicer when compiled. Place
  each fab task onto a separate line. This makes it easier to scan through the
  list of tasks, which is the most important part of the sample commands.
* Make some functional changes to sample commands. Add environment variables to
  the sample CDN installation command, and remove `.pub` from the sample `fab
  -i` command.

Update several docstrings in `fabfile.py`. Make it clearer what environment
variables can be specified and what they affect.