Introducing monitoring / bugfixes

• introducing unison monitoring - thank you a lot https://github.com/ricmatsui - this is yet off by default and will become on by default later, see #521 - also see this example for a little usage explanation https://github.com/EugenMayer/docker-sync-boilerplate/tree/master/default_monit
• various bugfixes, thank you a lot to the contributors, see https://github.com/EugenMayer/docker-sync/milestone/31?closed=1

This release includes references new unison images for native_osx and the unison strategy