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
By specifying a stage, it evaluates anything in that particular stage's file. So in the default case, it [sets a couple of values](https://github.com/endoze/mina-multistage/blob/c11bd2188b19e8e30d70a8dab72964e44a606e23/lib/mina/multistage.rb#L36-L40) to empty strings.