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
This shouldn't get moved out of the top level namespace. The idea is that you can use this command for more than deploying your app. Currently it supports `mina <command> <stagename>` so you could do something like `mina deploy staging`. This is similar to how capistrano multistage works in the 2.x branch of capistrano. I think better documentation would make this more clear.