supporting tags in updating a monitor

lib/api/monitor.js

@@ -137,6 +137,7 @@ function getAll(options, callback){
  *    optional, an object containing any of the following
  *    * name: the name of the monitor
  *    * message: the message for the monitor
+ *    * tags: a list of strings as tags to associate with the monitor
  *    * options: an object, to see available options please see the [monitor create](http://docs.datadoghq.com/api/#monitor-create) docs
  *  callback: function(err, res)
  *example: |
@@ -172,6 +173,9 @@ function update(monitorId, query, properties, callback){
         if(properties.message){
             params.body.message = properties.message;
         }
+        if(properties.tags){
+            params.body.tags = properties.tags;
+        }
         if(typeof properties.options === "object"){
             params.body.options = properties.options;
         }