Changes to data model, exam review UI, google sync, GPS, slide editor

Substantial changes to data model (now has an exam->slide->image
structure) and associated UI changes in exam editor, slide editor
(new), and slide review. Added two-way sync to google drive via JSON
serialization/deserialization. Added GPS tagging and map viewer. Added
function to add additional slides to an exam. revised exam list UI and
added sync indicator.