Mac zip disable

This pull request adds functionality to detect that the user is running on a Mac, and not offer the "download as ZIP" option.  It uses django-user-agent (and dependent stuff) to do the hard work.

There is currently a bug in the ua_parser code that is causing it to not find the default "regexes.yaml" file in the egg.  So I've copied that file into mytardis, and used a settings hack to tell ua_parser where to find it.  There's a pull request against ua_parser to fix this, but discussion has stalled.  So I think we should go ahead anyway.