Define Applicative instances for Box/Id

This silences GHC 7.8's AMP warning. See

  http://www.haskell.org/haskellwiki/Functor-Applicative-Monad_Proposal#Future-proofing_current_code

for more details
Update changelog
Merge pull request #3 from hvr/fix-amp-warnings

Define Applicative instances for Box/Id