Added support to virtualenv

Adding the name of the virtualenv at the beginning of the prompt.

Now it looks like the attachment.
![prompt](https://f.cloud.github.com/assets/59826/180349/a46d9ca0-7c2d-11e2-84a7-13d85e3b3cf3.png)

I don't know if it deserves a new color background, probably it does.