#57 should detect stale PID file

[Jeff Ahrenholz]

during startup, hip should detect if a PID file already exists but the hip process is not actually running, and print a message such as "warning: removed stale PID file since process 12345 was no longer running"; current behavior is just to exit with an error

also the location of /usr/local/var/run/hip.pid is slightly awkward, probably OK to just store it in /var/run/hip.pid