Avoid possible exception in LayerExternal model representation #178
Use django-hstore schema definition instead of crappy REQUIRED_FIELDS:
 * leverage django-hstore new reload schema feature
 * get rid of field_mapping field and use one HStore field only
 * get rid of REQUIRED_FIELDS and use SCHEMA definition from django-hstore
Cleaned up synchronizers #178
Improved UX in LayerExternal admin #178
Configuration keys are reloaded when synchronizer is changed
Removed olwidget dependency #152
Added django-leaflet widget to GeoModelAdmin #152
Removed redundant map settings in favour of LEAFLET_CONFIG #152