Name the versioned entry point after the major version of python only

This is what I'm doing in Debian to simplify our packaging. The approach is described in #943 and follows the same recommendations as [PEP-394](http://www.python.org/dev/peps/pep-0394/):

`pip` is the default pip - cPython 2.x (unless you are in a Python 3 / PyPy virtualenv)
`pip2` is cPython 2.x
`pip3` is cPython 3.x

If one really wants a specific python minor version, one can always use `python3.2 -m pip`