Instances for newtype wrappers in Data.Monoid

Hi,

I have written Serialize instances for all of the newtype wrappers in Data.Monoid;  they do the obvious thing, which is simply to unwrap the value inside the newtype and serialize/deserialize that.  I would appreciate it if you would merge this because in one of my projects I actually send Sum types over the wire.

I wrote this commit a couple of months ago, but I just rebased it before sending this pull request so merging it should be trivial.

Thanks!
Greg