Add mtl instances for MonadReader, MonadWriter and MonadState.
Fixed MonadWriter instance, added MonadError instance.

Fixed the implementation of the MonadWriter instance and moved liftCatchError
from Pipes.Lift to Pipes.Internal (reexported from Pipes.Lift) to implement the
MonadError instance.
Merged addition of unsafeHoist with mtl instances.
Merge pull request #59 from merijn/master

Add mtl instances for MonadReader, MonadWriter and MonadState.  Fixes #56