Rewritten for Capistrano 3

@relistan I've hacked about with this and ended up with a working Capistrano 3 version that behaves pretty much identically to the original. Unfortunately capistrano-spec still seems to be cap 2 only. I can't find a single cap 3 plugin that has tests, so reluctantly I've disabled the specs until I can either think of a way to test it, or a testing framework emerges.
Not sure why this is removed? It was there so that you can require it by the same name as the Gem, which is a convenience that I think was useful.  Otherwise you just have to know that it's `capistrano/deploytags`.