Add a model object representing an author + a collection of

Subclass NSMutableDictionary to represent a collection of authors.
Because of Cocoa relying on class cluster pattern for NSDictionary and
NSMutableDictionary, our subclass is actually a decorator/wrapper
around an NSMutableDictionary object used for internal storage.
Move some logic to author model object

Move logic handling generation of 75px avatar URLs into the
author model object. Also generate twitter handles within
that object.
Update README

This fixes a part in installation instructions that was not updated.