Making views more useful.

 - Individual game's details are now shown properly outside of admin
   view.
 - The "last edited" field is now automagical.
 - Will now use markdown in "notes" and "purchase provenance" fields.
   Requires python-django-markupfield and markdown libraries.
   NOTE: this requires schema changes and calling save() to update
   rendered content.
 - Added a "formatted price" method to properly format prices according
   to the currency.