New muddle "quickstart"

The muddle documentation has now gained a "Quick start" section, at http://muddle.readthedocs.org/en/latest/quickstart.html. I've also added an introduction, which more-or-less matches the front page text on Google Code.

Change to behaviour of "$ muddle" in src/

I have just changed muddle so that the behaviour of a "bare" muddle
anywhere under src/ has changed.