Scala Maybe.
Inspired by Lift's Box, Haskell's Maybe and Scala's Option.
A few noteable characteristics:
* `Maybe()` is a polymorphich constructor that Does The Right Thing, no matter the input.
* `get` is only provided for a `Just` value, not in the general `Maybe` API.
* `map` on a `Just` Does The Right Thing, even if the calling function explodes.