Add WriterT Monoid

[yilinwei]

I would like to add an implicit Monoid for WriterT. I've copied the implementation from scalaz here.

I would like to add some tests for the commit but am having a little trouble understanding how the tests work and should be structured. My initial attempt is here where I've just made sure that the Monoid and Semigroup can be resolved.

[adelbertc]

You'll also have to test the Monoid laws for it

[yilinwei]

I had some issues finding them at first (which was what I meant to ask) because I didn't realise they were under GroupLaws in kernel.

I've now added the test here.