Derive `Read` for newtype decls in Haskell backend

This relates to BNFC/bnfc#84 and comment https://github.com/BNFC/bnfc/pull/83#issuecomment-39684692. Currently the definition for Ident is:

    newtype Ident = Ident String deriving (Eq,Ord,Show)

This commit changes this definition to:

    newtype Ident = Ident String deriving (Eq,Ord,Read,Show)

This allows users to translate Ident's from one language to another,
e.g:

    LangX.IdentExp ident -> LangY.IdentExp (read (show ident))

One caveat is that is that ther newtype definitions will also derive
Read, though I can't comment on the significance of this?