Add deploy:<stage> tasks & Deploy staging by default & Refactoring

* Add `deploy:<stage>` tasks
* Deploy staging with default `deploy` task
* Rename `multistage:create_stagefiles` -> `mutistage:init`
* Remove `config/deploy/development.rb` stage file
I've never seen `development` stage before. Seems like [capistrano-multistage](https://github.com/capistrano/capistrano/wiki/2.x-Multistage-Extension) has only `production` and `staging` stages too.