Add in padUsersCount method and API call

The PadMessageHandler objects now have a new API call associated
with them. I'm sure that's a funny place to put it, but the
pad2sessions object in that file seems to be the only place user
counts are stored!

Anyway, I hope this is helpful. I know it would be for me :)

src/node/handler/APIHandler.js

@@ -65,7 +65,8 @@ var functions = {
   "getPublicStatus"           : ["padID"], 
   "setPassword"               : ["padID", "password"], 
   "isPasswordProtected"       : ["padID"], 
-  "listAuthorsOfPad"          : ["padID"]
+  "listAuthorsOfPad"          : ["padID"],
+  "padUsersCount"             : ["padID"]
 };
 
 /**