Gromacs, Git, and README

We try to keep the newest versions of formulas around. If we need to continue to support older versions (like python 2 and python 3 in parallel) I'll create two formulas python27_formula.rb and python34_formula.rb for example. Otherwise we want the newest version. If we need to perform a one off installation of an older version I use git to checkout an older revision of the formula. I've been meaning to archive older formulas in a separate repo similar to what homebrew does: https://github.com/Homebrew/homebrew-versions