Setup Spinner class as React component, implement spinner into login

Here's my first stab at implementing the Flux pattern in the app. I've used `login.coffee` as my guinea pig...all I'm trying to do is show a Spinner at the start of a `ServerRequest`, and hide it when one of the `done/error` events methods are fired.

It looks like it works, but I'd just like to make sure I'm doing it all correctly. I'm not entirely sure that I'm accessing the `store` properly in the `login` view.

Thanks.