Fixed a bug with req count decrementing, wrapped in provider

I was trying your handy interceptor but I noticed the request count was going negative.  The bug was due to decrementing the request count even if there were requests in the buffer.  I made a change and it seemed to work.  I also wrapped it in a provider so you could configure the outstanding request limit.

Sorry, but I didn't update the coffee script (I really need to learn coffee)