Added the support for keep-alive ping from the host,
when the caller does not call regularly ping method, the host will
shutdown itself after a minute of missing ping.

This is crucial in my scenario where I need to be completely sure
that I do not left plugin host opened if the main application
chrashes or is shutdown unexpectedly.
Merge branch 'master' of https://github.com/alkampfergit/BaktunShell