Added Revisioning

Added second table ‘evreg_eventstation_revision’ to schema.  Added
foreign key, ‘revision_id’ to ‘evreg_eventstation’.  Moved fields from
base table to revision table:  name, description, spots, guides, event,
owner, status, visibility.  Added foreign key, ‘esid’ to revision table.

Added revision entity key to _entity_info().

Added menu item to access revisions.

Added function as page callback: event_station_view_revision