Solve SearchCriteria inheritence problem.
Rename DataEditor to XMLEditor. Add multiple inheritence from it.

JSSObject and JSSObjectTemplate now also inherit XMLEditor.

Move duplicate methods __repr__ and _indent to XMLEditor.
Move _set_bool to XMLEditor (and rename to indicate public).
Change JSSObject and JSSObjectTemplate to Element. Fix appropriately.

All JSSObject children and JSSObjectTemplate children now inherit the
type of XMLEditor they want. At this point there is only XMLEditor, but
that will soon change.
Refactor XMLEditor path/element handling code.
Implement GroupEditor.

This is tricky. I chose to use GroupEditor for both computer and mobile
device groups. I could have further subclassed it, but chose not to.
Fix test for new criteria handling.
Changed my mind. Subclassed GroupEditor.
Add PolicyEditor. Migrate Policy and PolicyTemplate to it.
Add PackageEditor.
README update.