Events.list, allow stripe to populate with defaults

* Entering a null parameter or excluding one like ```stripe.events.list(100, func)```  shouldn't result in a request with  ```count=100&offset=&type=``` which doesn't work the way it should, but ```count=100```, allowing stripe to populate with defaults properly. 

* Added ```events.list()``` parameters which I needed

* Added the ```type``` parameter to normalize arguments logic

Thanks for writing the library, very handy