Add support for PID file

Use case: I use the monit daemon on my server to make sure the production services stay running; this requires having a PID file for each service; monit then knows what the process ID is and can check whether it's up, and take appropriate action.

The attached version of fisheyectl.sh places the pid file at the location specified by the FISHEYE_PID environment variable (default FISHEYE_INST/var/fisheye.pid). It also uses the pidfile to check for a running instance of Crucible, and prevents the service from being started twice.

I'm putting my modifications in the public domain so that this can be added to the shipped fisheyectl.sh. Can you review and include in a future release if appropriate? Credit would be appreciated but is not required.

Thanks!