Diagram documentation

:warning: Not ready to merge :warning:

I while back, I came across these diagrams: https://github.com/raimohanska/bacon.js/wiki/Diagrams.

A visual explanation of the various operations would be quite helpful to newcomers, particularly since the terminology and RAC's implementation code can be intimidating to the uninitiated.

At the time of creating this PR, only `+[RACSignal merge:]` and `-[RACStream flattenMap:]` have been diagramed, and are shameless knock-offs of the bacon.js originals with the addition of showing completion.

If there's interest in having visualizations like this added to the documentation, I'd be happy to continue to work on these. We can make a list of which are most commonly misunderstood, and I'll work off that.

As will be obvious, I'm not a designer, so if someone else is better at this, I'll happily defer.

The diagrams are made using Gliffy.com, the same tool used in the original bacon.js diagrams. It can export SVG, but right now these are PNG. If someone wants to suggest an alternative tool that will be better for ongoing maintenance of the diagrams, I'm all ears.