Add an option to the io_loop fixture to configure ioloop maker

[eugeniy]

Needed for apps that rely on IOLoop.instance()

Currently, IOLoop() is created, this should be overridable.

[holljen]

Yes please, I'd like this. I'm using zmq in addition to tornado and it relies on IOLoop.instance(). I've solved it by calling io_loop.install() but it isn't very pretty :)