Added mapK to StateT
Made the type of mapK on StateT more general.
add eitherTPlus, eitherTMonadPlus. fix EitherT#filter, EitherT#withFilter

https://groups.google.com/d/msg/scalaz/NCiRRxLjanM/zArkn18vScgJ
https://github.com/ekmett/either/blob/v4.0/src/Control/Monad/Trans/Either.hs#L147-L163
EitherTTest