Updated code to modern Objective-C syntax (literals)

Objective-C literals have been out for a while and iOS community generally agrees that they account for a better, more readable code ([reference](http://nsscreencast.com/episodes/27-new-objective-c-syntax)).

This PR has been simply created by using Xcode's `Edit -> Refactor -> Convert To Modern Objective-C Syntax` feature (and couple of manual edits, see below).

Xcode gave me some warnings in `NSNumber (KiwiAdditions)` category, I had to convert couple of methods manually there (see [in-line comment](https://github.com/allending/Kiwi/pull/210/files#r2871162)). Would be great if someone with more low-level C experience could take a look and see if I didn't screw anything up.