Redefine `ReadM` as `ReaderT String Except`

`ReadM` now includes the `String ->` bit.  This makes it less awkward to define
and compose readers.

This commit also makes `ReadM` work for argument readers (#106).
Bump version to 0.11.0
Test reader failure in argument
Reverse `Monoid` instance for `ParseError`

Later errors should trump previous ones
Remove dead method setParser of MonadP
Merge branch 'readm-refactoring'
Update CHANGELOG