Added one_shot option

Hi!

First of all, thanks for sharing this great library :)

I added option 'one_shot' which removes response object from _urls after first call.

With this option, you can test functions that makes several requests to one URL. For example, it can be function that checks given URL until any data becomes available, or functions with some complex error handling ("if remote API says it is busy, wait and make one more request").