add priority, fixes #29

Author: Alex Bagehot

lib/mapping.js

@@ -377,7 +377,12 @@ var options = {
       param: 'string',
       info: 'string to hash test to a specific test agent. tester will be picked by index among available testers'
     },
-
+    'priority': {
+      name: 'priority',
+      api: 'priority',
+      param: 'string',
+      info: 'change test priority'
+    },
     // Undocumented/experimental/transitent/deprecated
     'noads': {
       name: 'blockAds',