add .mapToTrack() to explicitly enable default mapping to .track()

closes #36 

adds `.mapToTrack()`, so we can easily map `.page()` calls to `.track()`.
the reason it's called `mapToTrack()` is because we might want to map `.screen()` calls later on ?
would be awesome if we can get away with `pageToTrack()` for now though...

cc @calvinfo