Added PopStateEvent

[coot]

This adds PopStateEvent which is in HTML5 spec.

There is no way of creating events, is this on purpose, or would you accept a PR which adds methods to create events?

[coot]

It would be nice to also include support for PopStateEvent.state property. I think we should reflect state type on the type level with:

foreign import data PopStateEvent :: Type -> Type

Would that be fine?

[garyb]

The missing create event functions are not for any particular reason, they're just missing 😉

I don't think we can do that unfortunately with the state type coming from PopStateEvent, since there's no way we can guarantee that the event is carrying state of the type we expect - we can do that when creating an event, but when it arrives from the DOM we'd just be unsafeCoerceing it pretty much.

[coot]

You're right. At least we can add readState :: forall eff. PopStateEvent -> Eff (dom :: DOM | eff) Foreign.