Make {pip,easy_install} a variable

On some systems there are multiple versions of `pip' and `easy_install'.  For
instance, on Arch Linux, there exists `pip' (python 3.x, provided by
`python-pip'), `pip2' (python 2.x, provided by `python2-pip'),
`easy_install-3.3' (python 3.x, provided by `python-distribute'),
`easy_install-2.7' (python 2.x, provided by `python2-distribute').  Hence, these
should be variables!