Make executable-find {pip,easy_install} a variable

`pip' and `easy_install' may have different names depending on the system.  For instance, on Arch Linux, there are multiple versions of Python in the repositories.  There exists python-pip (python 3.x, `pip'), python2-pip (python 2.x, `pip2'), python2-distribute (python 2.x, `easy_install-2.7'), python-distribute (python 3.x, `easy_install-3.3').  Hence, these should be variables!
Merge branch 'master' of https://github.com/eqyiel/elpy