Add port select support for pip and virtualenv

[cdeil (Christoph Deil)]

Would it be possible to implement the possibility to select a default pip and virtualenv? E.g.

sudo port select --set pip py27-pip
sudo port select --set virtualenv py27-virtualenv

Just like for python, ipython, cython or nosetests this would be very useful. pip and virtualenv are quite frequently used e.g. in shell scripts written by others to install software and if only pip-2.7 is on the PATH they fail. I would prefer setting the default version via Macports than via symlinks or aliases.