Added common python headers, so clustergit is ready for redistribution

Hi!

I really love your project, and I would like to use it in my own project(https://github.com/Smarties89/ses). So in order to credit you guys, I added typical python headers for crediting, license, description etc.

If you are interested in what I am going to use it for: ses(Simple Execution System) is a command line tool that executes snippets from central repositories. This way you only need to install one tool to run snippets. This way I import clustergit to my central repository, and people can run it with **ses run clustergit <cmdargs>**, without the need to install clustergit and update it.