Display map widget custom attribution

Fixes #87

ckanext/spatial/templates/spatial/snippets/map_attribution.html

@@ -2,6 +2,8 @@
 href="http://openstreetmap.org/copyright">OpenStreetMap</a> contributors</div>
 {% if map_config.type == 'mapbox' %}
   <div>Tiles by <a href="http://www.mapbox.com/about/maps/">MapBox</a></div>
+{% elif map_config.type == 'custom' %}
+  <div>{{ map_config.attribution|safe }}</div>
 {% else %}
   <div>Tiles by <a href="http://www.mapquest.com">MapQuest</a></div>
 {% endif %}