Use Process class to spawn rackup

Ruby 1.9 and later comes with better support for spawning child processes (like rackup). I've switched Guard::RackRunner to use the Process class instead of direct system calls. This allows guard-rack to not have to bother with PID files.

This is not backward compatible with Ruby 1.8. If this is a deal breaker, I can work on adding back the PID file support.