Continued unit test development

PureLayout is the successor of UIView+AutoLayout which has been extensively and thoroughly tested and proven on iOS.

The biggest difference between iOS and OS X is the inverted Y axis between the two coordinate systems.

For anyone using this project on OS X, I would appreciate feedback from your additional usage/functional testing to validate that there aren't any issues around the flipped Y axis.

As the unit test suite continues to be built out, coverage will improve on that front as well. (If you are interested in contributing some unit tests, that would be greatly appreciated!)