Also fixes the problem where pipelines were considered a single subshell.

* Checks now use Writer monad instead of State

* Parser no longer emits notes unrelated to parsing.

* All checks are now passed a parameter value, containing shell type,
  map from notes to parents and such. This eliminates recalculation
  and removes the need for a special group of parent examining checks.

Also, do it recursively.

Added MacPorts cabal install information to README

Capital P on MacPorts

Added cabal setup instructions for Mac OS X to README

Un-Markdown text

expanded cabal file

The manual is mainly a copy/paste from the current documentation. It is
builded with pandoc, and written in markdown.

closes #50