Refactoring capistrano 3 tasks

I hope you like it:

1. On **restart** method: 
  * Placed invoke's methods outside of **on** block, to avoid verbose error. Calling invoke inside an on block, if the called method has a on block.
2. On **start**, **stop**, **quite** methods:
  * Using the SSHKit command_map for each method, and keeping the alternative to use free command.
  * Fixing the pid_file path.

Thanks.