Various RAC style/idiom fixes

First of all, great talk! It's always awesome to see people using RAC, and especially writing examples for others to learn from (we're not so great at that last part ourselves).

As I was watching the presentation, I noticed some things that I wanted to offer more idiomatic alternatives to. Each commit here is self-contained and hopefully self-explanatory, and a few include references to relevant parts of the [Design Guidelines](https://github.com/ReactiveCocoa/ReactiveCocoa/blob/master/Documentation/DesignGuidelines.md) for deeper rationale.

Hope this is helpful!