Fixed concurrent requests & added increment and decrement

Hi there,

Thanks for the nice library - I needed it to connect to heroku with the password.  I noticed a problem with how the "response" event was being handled.  Basically, if two requests hit the same server and were both waiting on a response - one response would be sent to both requests. 

I added a test case and fixed the code.  I had to change the client_test.js a fair bit to handle lists of callbacks, but I think it is fundamentally the same as it was.

Let me know if you have any questions.

Thanks,
Josh