Add ability to enable/disable layers

lib/carto/tree/layer.js

@@ -2,6 +2,7 @@
 
 tree.Layer = function Layer(obj) {
     this.name = obj.name;
+    this.status = obj.status;
     this.styles = obj.styles;
     this.properties = obj.properties  || {};
     this.srs = obj.srs;
@@ -23,6 +24,7 @@ tree.Layer.prototype.toXML = function() {
     return '<Layer' +
         ' name="' + this.name + '"\n' +
         prop_string +
+        '  status="' + this.status + '"\n' +
         '  srs="' + this.srs + '">\n    ' +
         this.styles.reverse().map(function(s) {
             return '<StyleName>' + s + '</StyleName>';