Nonnull/Nullable annotations for properties and methods

[oleganza]

Today Xcode 6.3 with Swift 1.2 is out and we can use new annotations: __nullable and __nonnull (see https://developer.apple.com/swift/blog/?id=25)

We should eventually annotate all methods and properties explicitly. Use of audited regions (NS_ASSUME_NONNULL_BEGIN etc.) should be used sparingly to avoid making mistakes or making the code hard to follow.