NF: Allow setting of timeout for execution of action commands

This uses subprocess.Popen, polling until `timeout` seconds has passed
or the command has exit. If the command has not exited, fail2ban then
sends SIGTERM, and if this is unsuccessful, SIGKILL.

The timeout can be changed for an entire action via action [Init]
options, or via jail.conf override, or fail2ban-client. The default
timeout period is 60 seconds.

Reference #60