update map view to include proper attribution for openstreetmap tiles

Previously, we were able to point to the  OpenStreetmap tiles rather than the default MapQuest tiles using the mapURL attribute in the map view configuration.  However, we weren't able to provide the proper attribution. @zepheiryan has modified exhibit to include a mapAttribution attribute, and I've modified the view configuration to use this.  A future enhancement might be to break these settings out so they can be derived from the settings.py, or so that multiple tilesets could be selectable in the editor.