Configure Capistrano and add Unicorn control tasks

This adds necessary Unicorn configuration in `config/unicorn.rb`, and
a corresponding set of Capistrano tasks to remotely
start/restart/shutdown the web server. Ex usage:

cap production unicorn:start
cap production unicorn:restart
cap production unicorn:stop
cap production unicorn:add_worker
cap production unicorn:remove_worker

See http://unicorn.bogomips.org/SIGNALS.html for excellent docs
on the various signals that Unicorn responds to.