Implements Reader Post Details Lazy-Fetch

Fixes #2152 

/cc @aerych (Thank you!)

Two tiny comments here for both of the class methods..
Traditionally we've put class methods near the top of the implementation file, above dealloc and init. 

The other, and this is super nitpicky, but `[ReaderPostDetailViewController postDetailsWithPost:]` reads a little weird to me. I'm not sure exactly what post details means. How do you feel about something like `[ReaderPostDetailViewController detailControllerWithPost:]` so its clear from the method name that its a controller factory.