Splat url helpers

Closes https://github.com/phoenixframework/phoenix/issues/527

@josevalim - Still couldn't figure out what to do with `NAME_path(conn | endpoint, ...args)`. At first I thought I'd use path_info in `%Conn{path_info: path_info}`, but it will only work for a conn that has the correct path_info. What in conn do I need to use for `NAME_path`?