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
It's a common scenario: develop -> deploy to staging -> check all is ok -> merge to production -> deploy to production. I think we deploy to staging much more often.