Add support for "else" symbolizers

When a style has no rules, the "else" symbolizers apply.  When a style has rules and none of them apply to the given feature, the "else" symbolizers apply.  Note that this is different than default symbolizer properties that might be merged into all symbolizers (as in OL2) - I don't think we should support that.

examples/topojson.js

@@ -23,21 +23,19 @@ var vector = new ol.layer.Vector({
     url: 'data/topojson/world-110m.json',
     parser: new ol.parser.TopoJSON()
   }),
-  style: new ol.style.Style({rules: [
-    new ol.style.Rule({
-      symbolizers: [
-        new ol.style.Fill({
-          color: '#BADA55',
-          opacity: 0.5
-        }),
-        new ol.style.Stroke({
-          color: '#FFF',
-          opacity: 1,
-          width: 1.5
-        })
-      ]
-    })
-  ]})
+  style: new ol.style.Style({
+    symbolizers: [
+      new ol.style.Fill({
+        color: '#BADA55',
+        opacity: 0.5
+      }),
+      new ol.style.Stroke({
+        color: '#FFF',
+        opacity: 1,
+        width: 1.5
+      })
+    ]
+  })
 });
 
 var map = new ol.Map({