Adding tests

**Not ready for merge!** - Pull request opened for discussion only at this stage

I've started adding tests to UIView+AutoLayout. I'm using [Specta](https://github.com/specta/specta)+[Expecta](https://github.com/specta/expecta) for a test / matcher framework. I've also slightly restructured the demo project.

So far I've added tests for:

- `autoLayoutView` convenience method
- Constraining the size of views
- Centering views

My approach so far has been to create constraints on views using the category and then to verify that the expected constraints have been added. However, it may also be good for me to add assertions based around the frames of views after a layout pass. My only issue with this is that it seems to me that it would really be dependent on the workings of AL's layout engine, rather than directly testing this class.