Send/recieve JavaScript native Date through a port

I thought I'd quickly add a native `JSDate` to/from `Date` for ports since it looked very easy to do.

However, before I spend any time I thought I'd first check whether this looks like the correct approach? If so I'll finish this up.

I presume that https://github.com/elm-lang/elm-compiler/blob/master/tests/test-files/good/Ports.elm should get a test case?