draft for a Validation DSL
andAlso/andWith for Checker

With that, Checker might be an Applicative, but I'm not yet convinced of that.
Its laws will have to be checked (no pun intended).
tuned
check laws for Checker
Merge branch 'topic/checker'
use scalaz-specs2
update README
add new submodules to aggregate
prepare release: include sbt-release and sbt-pgp
README for undo