Validation.of
Refactored Success and Fail to a single validation class.
Added append to NEL and fixed size.
Implemented headMaybe on List
Implemented join on all monads.  Map2 is now available on all monads.
Map now derived where possible.
Added conversions between Either, Validation and Maybe.
Added orElse to Maybe
Updated IO examplem to use NEL in Validation
Added fold to Maybe
0.6.6 release
Merge branch 'release/0.6.6'