Auto-release plugin

I wrote a plugin that does the "git flow release" thing automatically: creating tags, git flow release start, bumping versions in the build file... all that stuff. If you like it, let's get this into algebird and scalding.

I've got it deployed to maven central; it looks like to resolve properly against any 0.12.x version of sbt (not just 0.12.0), I need to get the plugin into the sbt community ivy repo. I've got a ticket out to do this now. For now, all this means is we'll get a warning when running sbt commands (if you're not on 0.12.0).