Launch Unicorn as the runner user (defaults to "app").

In addition to launching Unicorn, the runner user must:
- delete the pidfile (since that user made it)
- send kill -0 to test the process
- kill Unicorn
Update Readme and add :unicorn_use_runner option.
Merge branch 'try-runner' of github.com:BMorearty/capistrano-unicorn into try-runner

Conflicts:
	lib/capistrano-unicorn/capistrano_integration.rb