Added ch.gis.widgets.GeoAdminWidget

Based on recent work on Django GIS widgets, this is an example of subclassing the new BaseGeometryWidget to provide a custom map widget. geo.admin.ch is the national reference wrt mapping quality in Switzerland (data from Swiss Federal Office of Topography). No urge, we can wait for 1.6 release before merging this. Note that this is the first template in django-localflavor, I hope it is fine.