Added the OSM [ref](http://wiki.openstreetmap.org/wiki/Key:ref) tag of the runway/taxiway so that they can be displayed in styles.

layers/aeroway/aeroway.yaml

@@ -6,6 +6,7 @@ layer:
       other airport related polygons can be found in the **aeroway** layer.
   buffer_size: 4
   fields:
+    ref: The OSM [`ref`](http://wiki.openstreetmap.org/wiki/Key:ref) tag of the runway/taxiway.
     class:
       description: |
           The original value of the [`aeroway`](http://wiki.openstreetmap.org/wiki/Key:aeroway) tag.
@@ -18,7 +19,7 @@ layer:
       - apron
   datasource:
     geometry_field: geometry
-    query: (SELECT geometry, class FROM layer_aeroway(!bbox!, z(!scale_denominator!))) AS t
+    query: (SELECT geometry, ref, class FROM layer_aeroway(!bbox!, z(!scale_denominator!))) AS t
 schema:
   - ./layer.sql
 datasources: