bobthefish: added promt support for virtualfish

It's a promt for https://github.com/adambrenecki/virtualfish

![screenshot](http://i.imgur.com/lmndI0k.png)

`◰` means virtualenv (unfortunately there is no "python" or "snake" unicode symbols in common fonts), `³` means the version of python.

UPD: sadly I didn't know about this PR: https://github.com/bpinto/oh-my-fish/pull/144 I don't know why was it closed.
@adambrenecki 
I thought about `pypy` and `python3`. Is it [ready](http://pypy.org/py3donate.html)? I thought no, so I've done some simplification.

On the other hand, there are other `python` interpreters (jython). That means that probably I would get both: version and type. Which variant do you like more: `2ᵖ` or `p²` for pypy version 2? Or something else? I think that the first one is better, because in theory programs written in `python2` should be compatible with all `python2` interpreters, but `python2` and `python3` are incompatible in many aspects. So, `2` is more important, than `p`.