Update and rename TODO to TODO and Roadmap

I am not a developer or anything, but I think this will make people understand more Tox schedule, how soon they are from a new release (alpha, beta, gamma, stable), etc.

Most likely the things I put in each category aren't completely right, so feel free to correct them if that's the case, developers.

Also, I think there should be a release candidate somewhere there. Not sure though. Just wanted to get this started.