Add support for Fossil SCM

Add the possibility to follow changes made in a [fossil](http://www.fossil-scm.org/index.html/doc/trunk/www/index.wiki) repository. I try to conform with the other SCM implementations. I choose the UTF8 symbol ⌘ for the prompt. It's the universal geographic point of interest symbol and I thought it is a good choice as fossil-full locations are good points of interest.

I've also commented one line to avoid a long running issue. If it's not pertinent, you can of course just cherry-pick the two first commits to grab only the fossil related stuff.

Happy new year everyone.