Refactor using capistrano-unicorn as an example

I noticed that I was getting errors during deploy, because unicorn:restart rake task was looking for pid file when it wasn't created yet (e.g. first run).
I decided to port most of the [capistrano-unicorn](https://github.com/sosedoff/capistrano-unicorn/) code and it runs fine now.
(I removed parts about configuration and user-related code, because I'm not sure how to address it and it works for me without it anyway)