Asynchronous API for map#getFeatureInfo

This change allows us to use feature info services on the
server. It will also be useful for the GetFeature control to
get results from a WFS.

src/objectliterals.jsdoc

@@ -5,6 +5,20 @@
  * @property {ol.ProjectionLike} projection Projection.
  */
 
+/**
+ * @typedef {Object} ol.GetFeatureInfoOptions
+ * @property {ol.Pixel} pixel Pixel coordinate relative to the map viewport.
+ * @property {Array.<ol.layer.Layer>|undefined} layers Layers to restrict the
+ *     query to. All layers will be queried if not provided.
+ * @property {function(Array.<ol.Feature|string>)} success Callback for
+ *     successful queries. The passed argument is the resulting feature
+ *     information.  Layers that are able to provide attribute data will put
+ *     ol.Feature instances, other layers will put a string which can either
+ *     be plain text or markup.
+ * @property {function(Object)|undefined} error Callback for unsuccessful
+ *     queries.
+ */
+
 /**
  * Object literal with config options for the map.
  * @typedef {Object} ol.MapOptions