Allow optional Archetype field with required properties

Fixes #68 - see background on the issue there.

This will now skip Archetype's validation when there's only 1 empty fieldset.  In the screencast, you can see the validation doesn't kick in until the user adds a fieldset or enters data in any of the single fieldset's fields.

Screencast:  http://screencast.com/t/tEn0LOqRj

This does remove the ability to require at least 1 fieldset in an Archetype.  As a solution, we could either add a Min Fieldsets prevalue, and/or we can key off whether the property is marked required in Umbraco (U4-4432)