JSONP Callback Bug

Hey fxb,

I've been using this library for a project, and ran into a bug when making concurrent requests to the API. In particular, if calls happen too fast, the JSONP callback name may not be unique, resulting in missed callbacks. All I did was add a counter to help ensure uniqueness. 

Here is an example of code that illustrates the bug, and is fixed by the patch:

https://gist.github.com/742237

Clif