Add setStyle and getStyle to ol.Feature

As with vector layers and feature overlays, feature style can be set by calling
setStyle.  Calling getStyle returns what was passed to setStyle.  Internally, we call getStyleFunction.

src/ol/format/kmlformat.js

@@ -1448,7 +1448,7 @@ ol.format.KML.prototype.readPlacemark_ = function(node, objectStack) {
   } else {
     featureStyleFunction = ol.format.KML.makeFeatureStyleFunction_(style);
   }
-  feature.setStyleFunction(featureStyleFunction);
+  feature.setStyle(featureStyleFunction);
   return feature;
 };