be able to set the the reference

lib/carto/tree/reference.js

@@ -8,14 +8,19 @@
 
 
 var _ = require('underscore');
-var mapnik_reference = require('mapnik-reference');
+var reference = require('mapnik-reference');
 
 tree.Reference = {
-    data: mapnik_reference.version.latest
+    data: reference.version.latest
+};
+
+tree.Reference.set = function(ref, version) {
+    reference = ref;
+    tree.Reference.setVersion(version || 'latest');
 };
 
 tree.Reference.setVersion = function(version) {
-    tree.Reference.data = mapnik_reference.version[version];
+    tree.Reference.data = reference.version[version];
 };
 
 tree.Reference.required_prop_list_cache = {};