Fix up scrolling to handle Collection View with content inset

Under iOS 7, it's common for collection views to be laid out with a `.contentInset`, such that they appear underneath the nav bar. This renders the existing scrolling logic faulty since it:

- Considers a date which is presently underneath the nav bar to be visible
- Doesn't take the `contentInset` into account when deciding where to scroll to

I've switched the code around a bit to resolve these issues