Add surface field for highways

Keep surface field from OSM on highways, generalize it to two values:
"paved" and "unpaved".

This is a fix for #389 and a partial fix for #422.

layers/transportation/transportation.yaml

@@ -105,10 +105,16 @@ layer:
           value of [`indoor`](http://wiki.openstreetmap.org/wiki/Key:indoor) tag.
       values:
       - 1
+    surface:
+      description: |
+        TODO
+      values:
+      - paved
+      - unpaved
   datasource:
     geometry_field: geometry
     srid: 900913
-    query: (SELECT geometry, class, subclass, oneway, ramp, brunnel, service, layer, level, indoor FROM layer_transportation(!bbox!, z(!scale_denominator!))) AS t
+    query: (SELECT geometry, class, subclass, oneway, ramp, brunnel, service, layer, level, indoor, surface FROM layer_transportation(!bbox!, z(!scale_denominator!))) AS t
 schema:
   - ./class.sql
   - ./update_transportation_merge.sql