Reverse method on event readers.

Related to #87.
I've been thinking about this for a couple of hours now.

The reason I returned the concrete types is because I was (incorrectly) taking into account implementation details (e.g. Atom Feed pages).

Since the event reader types are iterators and implement `IEnumerable<T>` it's fine to also have the `Reverse` methods returning `IEnumerable<T>`.

That *is* also compatible with the Postel's law; we are returning what we actually are – `IEnumerable<T>`.

What do you think?