Add AsyncStringFeatureParser and use plain

XMLHttpRequest in the KML example

Since the content-type on github.io is application/octet-stream we need
to implement an Async string based parser interface in the KML parser.
Also use plain XmlHttpRequest in the example instead of jQuery Ajax since
the vector-features example also uses that.

src/ol/parser/featureparser.js

@@ -1,4 +1,5 @@
 goog.provide('ol.parser.AsyncObjectFeatureParser');
+goog.provide('ol.parser.AsyncStringFeatureParser');
 goog.provide('ol.parser.DomFeatureParser');
 goog.provide('ol.parser.ObjectFeatureParser');
 goog.provide('ol.parser.ReadFeaturesOptions');
@@ -56,6 +57,23 @@ ol.parser.StringFeatureParser.prototype.readFeaturesFromString =
 
 
 
+/**
+ * @interface
+ */
+ol.parser.AsyncStringFeatureParser = function() {};
+
+
+/**
+ * @param {string} data String data.
+ * @param {function(Array.<ol.Feature>)} callback Callback which is called
+ * after parsing.
+ * @param {ol.parser.ReadFeaturesOptions=} opt_options Feature reading options.
+ */
+ol.parser.AsyncStringFeatureParser.prototype.readFeaturesFromStringAsync =
+    goog.abstractMethod;
+
+
+
 /**
  * @interface
  */