For the `IPublishedContent` implementation, added support for `GetIndex()`, so that extension methods, such as `IsFirst()`, `IsLast()`, etc will work as expected.
Unfortunately we wont be able to support the `Siblings()`, (or `FollowingSibling`, `PrecedingSibling`) extension methods, as these perform an internal query to the node's parent's children - which in the context of Archetype data is not possible access.
Added unit-tests to support this enhancement.
leekelleher
68c2f55111