Simplify PropertyValueConverter

* Ensure the converter returns an empty `Archetype` instead of `null`, to avoid the need for null checks in Views

* Make the `Archetype` model enumerable to avoid the need for the extra property

* Removes (mark internal for now) the `Fieldsets` property to encourage using the class

**This is a breaking change!  **
You need to remove `.Fieldsets` from any of your Views.  Ex:

Old:
````c#
if (Model.Content.HasValue("people"))
{
    foreach (var person in Model.Content.GetPropertyValue<Archetype>("people").Fieldsets)
````

New:
````c#
    foreach (var person in Model.Content.GetPropertyValue<Archetype>("people"))    
````

Fixes #66