Enable Support for Shared Deployment Environment

Allow `sudo` to stop resque worker instances.  In an environment where
multiple people can deploy, `sudo` is required to allow the deployment
to kill the previous instances.

By default `sudo` is not used.  Enable with:

    set :enable_shared_users, true

Replaced the `stop_command` with Ruby code.  The shell code using `sudo` would hang on the `kill -0` test.