Add Data.Conduit.List.iterM

This Conduit simplifies performing a monadic action for every value
generated by a Source, and passing the value on as-is.
Add QuickCheck tests for iterM behaviour and mapM equivalence

Requested in pull-request review.

See: https://github.com/snoyberg/conduit/pull/68#issuecomment-8113128
Merge branch 'iterm' of https://github.com/NicolasT/conduit
Merge iterM to test suite