Proposal to solve the `EitherT` problem.

[Roman Cheplyaka]

* Gabriel Gonzalez <gabriel439 at gmail.com> [2013-06-16 14:52:46-0700]
> * Approach 2: Add `EitherT` to `transformers` alongside `ErrorT` and
> have them both implement `MonadError`.

I already submitted the proposal to merge EitherT into transformers [1],
which enjoyed universal support, barring some implementation details.

So why not just bring that proposal to its conclusion?

[1]: http://www.haskell.org/pipermail/libraries/2012-December/019027.html

Roman