Delete pid_file after the process ends

Since the process should be exited after sending
an INT signal, the pid file can be deleted. That
makes it easy to add a simple status method,
which checks if the pid file is there.

Additionally the pid file will be deletet if an
SIGINT or SIGTERM arrives.

A future improvement could be the monitoring of
the process and delete the pid file if the process
is killed.
Don't delete the pid file if it doesn't exist
Interrupt: Check also if @pid_file is set
Merge pull request #3 from phortx/master

Delete pid_file after the process ends