Add virtualenvwrapper and its dependencies

This is work in progress so please do not merge it.

I know that virtualenvwrapper is not really necessary in NixOS but there are some situations where it could be useful (for example work related projects which use virtualenvwrapper functions in helper scripts...).

Some points need to be fixed:

- pip requires a valid $HOME (I think to cache or something like that). Is there a proper way to handle this?

- A virtualenvwrapper script (called virtualenvwrapper.sh) tries to use the programm "which", which isn't available. Should I patch the shell script or add a runtime dependency on "which" ?

- Python could not import the module virtualenvwrapper.hook_loader (this is the error message when using virtualenvwrapper scripts). I think this is the desired behavior, right? NixOS's python knows nothing about virtualenvwrapper. Is there a simple workaround? Sure, we can use python27Full with virtualenv* in extraLibs but this is a bit complicated. If a user installs virtualenvwrapper it should work out of the box.

If you have any questions please let me know