Simpler type for ol.events.Key

Instead of having `ol.events.Key` be a listener object or an array of listener objects, it should be less error prone to have it just be a single listener object.

To avoid using too many functions with multiple return types, the `ol.events.*` functions for registering and unregistering listeners no longer accept an array of event types (and only a single key is returned when registering).

To make it convenient for users to register multiple listeners at once, the `observable.on()` method accepts an array of event types.  Internally in the library, we should use the less risky `ol.events.listen()`.

src/ol/interaction/snapinteraction.js

@@ -168,11 +168,13 @@ ol.interaction.Snap.prototype.addFeature = function(feature, opt_listen) {
     segmentWriter.call(this, feature, geometry);
 
     if (listen) {
-      this.geometryModifyListenerKeys_[feature_uid] = geometry.on(
+      this.geometryModifyListenerKeys_[feature_uid] = ol.events.listen(
+          geometry,
           ol.events.EventType.CHANGE,
           this.handleGeometryModify_.bind(this, feature),
           this);
-      this.geometryChangeListenerKeys_[feature_uid] = feature.on(
+      this.geometryChangeListenerKeys_[feature_uid] = ol.events.listen(
+          feature,
           ol.Object.getChangeEventType(feature.getGeometryName()),
           this.handleGeometryChange_, this);
     }