Capistrano 3

Add support for Capistrano 3

Use:
require 'airbrake/capistrano3'
after :"deploy:restart", "airbrake:deploy"

No tests added. All ears on how to get a Repo name. Doesn't seem like Capistrano has that feature anymore.

No logic was changed, and there is duplicate code from capistrano.rb

Feel free to edit / make better / trash.