Add layerName option for WMSGetFeatureInfo format

This option allows the format to read only features of a
given layer. This is useful if you wish to make a single
query to a WMS server with multiple layers in it.

examples/getfeatureinfo-layers.js

@@ -0,0 +1,24 @@
+goog.require('ol.format.WMSGetFeatureInfo');
+
+$.ajax({
+  url: './data/wmsgetfeatureinfo/osm-restaurant-hotel.xml',
+  success: function(response) {
+
+    // this is the standard way to read the features
+    var allFeatures = new ol.format.WMSGetFeatureInfo().readFeatures(response);
+    $('#all').html(allFeatures.length.toString());
+
+    // when specifying the 'layers' options, only the features of those
+    // layers are returned by the format
+    var hotelFeatures = new ol.format.WMSGetFeatureInfo({
+      layers: ['hotel']
+    }).readFeatures(response);
+    $('#hotel').html(hotelFeatures.length.toString());
+
+    var restaurantFeatures = new ol.format.WMSGetFeatureInfo({
+      layers: ['restaurant']
+    }).readFeatures(response);
+    $('#restaurant').html(restaurantFeatures.length.toString());
+
+  }
+});