Good job! The point of this homework was to explore some challenges involved in developing a richer MVC client.
Very clean implementation, nice work.
You should definitely try implementing the optional features. There are quite a few things in there that are trickier than they appear and will prepare you to build more complex apps in the future.
Here's my checklist as I'm reviewing the submissions:
Code styling. The New York Times has a good summary of appropriate Objective-C coding conventions, you should take a quick look here.
Was the current user persisted across restarts?
Did you create a custom Tweet cell and properly set up Auto Layout constraints?
Did you augment the User and Tweet model with the required properties?
Date parsing and formatting is expensive, so consider caching in models when you can.
Does tapping on a Tweet take you to a detailed Tweet view?
Is the detailed Tweet view styled like the mock?
Did you add both standard and retina (@2x) image assets? A new feature of XCode is to use the Images.xcassets, it makes it easier to handle the standard and retina assets. If you only included standard assets, it will be scaled for retina displays which makes them a little blurry.
Ready for review: cc @nesquena @timothy1ee