iOS Improvements: Table View Source

This PR adds a number of helpful extensions to the UIKit code, extracted from [REDACTED].app

This adds:
* Reactive versions of common UIKit classes, such as UIImageView
* A way to bind ICommands to UIControlEvents
* A TableViewSource that attaches to a `ReactiveList<T>`

## ReactiveTableViewSource

This class is a pre-canned DataSource for ReactiveLists that you can attach to UITableViews - currently, support for editing or user reordering isn't supported, but multiple sections are supported. As you add items to your ReactiveList, ReactiveTableViewSource will automatically animate them in and out to match.