Add support of WMTS Get Cap read

This commit fixe #2721. It still lacks the possibility to create layer from the GetCap document. It also lacks the reading of TMS limits.

examples/wmts-capabilities.js

@@ -0,0 +1,8 @@
+goog.require('ol.format.WMTSCapabilities');
+
+var parser = new ol.format.WMTSCapabilities();
+
+$.ajax('data/WMTSCapabilities.xml').then(function(response) {
+  var result = parser.read(response);
+  $('#log').html(window.JSON.stringify(result, null, 2));
+});