Moved all onDelta tests into their own section.
Prevent cascading delete.
Send correct info via change event.
Clarifying comments.
Review feedback.
Merge pull request #604 from kadams54/fix-destroying-unit-1368556

Prevent unit/service deletes from cascading to machines.

Prevents cascading deletes from happening to containers/machines when a unit/service they host is deleted. Fixes these bugs:

https://bugs.launchpad.net/juju-gui/+bug/1368556
https://bugs.launchpad.net/juju-gui/+bug/1368573
https://bugs.launchpad.net/juju-gui/+bug/1368578