Add a parser for reading and writing KML.

This adds ol.parser.ogc.KML which can be used to read and write KML documents.
NetworkLinks are retrieved asynchronously.
Current caveats of the implementation are:

* LabelStyle not yet implemented. Missing support in renderers.
* When using shared structures the parser needs to be configured with
  dimension 2.
* We need a better way to disable fill, currently we use opacity as a
  workaround.
* We cannot really roundtrip documents, since some of the info is not
  preserved in the ol structures. But we can write out most of the important
  info.

src/ol/parser/featureparser.js

@@ -1,3 +1,4 @@
+goog.provide('ol.parser.AsyncObjectFeatureParser');
 goog.provide('ol.parser.DomFeatureParser');
 goog.provide('ol.parser.ObjectFeatureParser');
 goog.provide('ol.parser.ReadFeaturesOptions');
@@ -54,6 +55,23 @@ ol.parser.StringFeatureParser.prototype.readFeaturesFromString =
     goog.abstractMethod;
 
 
+
+/**
+ * @interface
+ */
+ol.parser.AsyncObjectFeatureParser = function() {};
+
+
+/**
+ * @param {Object} obj Object representing features.
+ * @param {function(Array.<ol.Feature>)} callback Callback which is called
+ * after parsing.
+ * @param {ol.parser.ReadFeaturesOptions=} opt_options Feature reading options.
+ */
+ol.parser.AsyncObjectFeatureParser.prototype.readFeaturesFromObjectAsync =
+    goog.abstractMethod;
+
+
 /**
  * @typedef {function(ol.Feature, ol.geom.GeometryType):ol.geom.SharedVertices}
  */