Rename `router` to `browser_router`

[Archmonger]

We need to match react-router naming conventions.

[rmorshea]

I'm not really sure our router implementation maps neatly onto what react-router is doing. Making this distinction might make more sense once we have a use_history hook (#8) since that seems to be where the behavior of the different routers diverge.

[Archmonger]

Will probably have to have a follow-up issue to think about how closely or how far we diverge from react-router's design.