Adding unit tests - work in progress!

Right, let's try this again...

I've reorganized the example project ever so slightly, and added the first batch of tests. I'll start working through the rest of the methods exposed via the header now - note this is **not ready to merge yet!**.

Previously there was some discussion over whether we should be testing constraints or testing frames - I've decided to test both just to be on the safe side. Each test generally has some assertions for the constraints created by a method, and the frames that result from a layout pass.