Implement nullable / nonnull attributes on parameters and properties

[joeljfischer]

Xcode 6.3 introduces attributes for parameters and properties that indicate nullability, mainly for Swift interop (EDIT: Also for Obj-C interactions!). We'll want to implement that into our APIs to help any Swift developers out there.

 #pragma clang assume_nonnull begin
     // ...
     -(void)registerNib:(UINib *)nib forCellReuseIdentifier:(NSString
*)identifier;
-(nullable UITableViewCell *)cellForRowAtIndexPath:(NSIndexPath)indexPath;
@property (nonatomic, readwrite, retain, nullable) UIView *backgroundView; // ...
#pragma clang assume_nonnull end

Adding nullability annotations to Objective-C APIs does not affect backward compatibility or the way in which the compiler generates code. For example, nonnull pointers can still end up being nil in some cases, such as when messaging a nil receiver. However, nullability annotations—in addition to improving the experience in Swift—provide new warnings in Objective-C if, for example, a nil argument is passed to a nonnull parameter, making Objective-C APIs more expressive and easier to use correctly.

[asm09fsu]

Merged into develop.