Fix issue #162 + improve nested validation highlighting

The fieldset.isValid property isn't necessarily initialized when the
view looks for it to determine fieldset validity. I've introduced a
validation method that counters this problem.
The validation method also ensures that any validation errors in nested
Archetypes are highlighted on the parent/root Archetype
Merge branch 'pr/157' into pr/164
Merge remote-tracking branch 'upstream/master'