We need some kind of build/install infrastructure

At the moment, all it would have to do is rewrite the shebang line at the top of git-imerge to cause it to be run with the correct Python interpreter for the target system.  Soon we probably want to move part of git-imerge to a library to make testing easier, and that will require a bit more structure in the project.

Something like setuptools is probably the way to go.
