Fix #212 - Implement InGroupsOf extension

This PR adds ```EnumerableExtensions``` to the ```Archetype.Extensions``` namespace in order to support ```InGroupsOf``` on any ```IEnumerable<T>```. Thus, if adding the namespace ```Archetype.Extensions``` to a view, one can use ```InGroupsOf``` on ```Fieldsets```, ```Properties``` and whatnot - e.g. ```myArchetype.Fieldsets.InGroupsOf(2)``` or ```myArchetype.Fieldsets.First().Properties.InGroupsOf(2)```