New theme: Serenity

Hello!

I just installed oh-my-zsh and played around. I had created a minimal theme to suite my needs, called 'serenity'. It will show the minimal information about the system (machine and actual dir) and has git integration.

I hope you will find it useful.

Regards,
Juan.

P.D: Sorry for my bad english.
P.D2: And this is my first pull-request, hooray!