Add FunctionalJava here too

Signed-off-by: Ricky Elrod <ricky@elrod.me>
Remove setters, FedmsgMessage is now immutable (but not pure, sadly)

Signed-off-by: Ricky Elrod <ricky@elrod.me>
Wrap sign() in IO and make it result in a (vaguely) more reasonable type (Either<Exception, SignedFedmsgMessage> when run. Someone please remind me who thought Java was ever a reasonable thing to use. I need to have a word with them.

Signed-off-by: Ricky Elrod <ricky@elrod.me>