Zimbra backup script improvements and obnam compatibility

I found your Zimbra backup script useful, but had to make several changes to make it suitable for my environment.  I implemented almost all of these as configurable options, so I'm submitting them with the hope that they may be useful to others.  The most significant is the option to using obnam instead of rsync as a backup utility.  Each commit represents one feature change (in case you want to cherry-pick) and the commit messages should be self-explanatory.

Let me know if you have any questions.