More interface-builder friendly.  Added nib-based demo.

In a similar vein to [TTTAttributedLabel](/mattt/TTTAttributedLabel) and [FXLabel](/nicklockwood/FXLabel), I've made some adjustments to this class to make it more friendly to power-users of Interface Builder.  It changes quite a lot about the internal workings of the class (most notably, the superclass is now `UILabel` instead of `UIView`, so you can set all the `UILabel` properties in IB) so there may be significant unintended side-effects.  It's hard to tell where the regressions are without unit tests, but from what I can tell, the demo project at least still behaves correctly.

I've created a second target in the demo project which replicates the original one except it has a barebones `viewDidLoad:` method and all labels and their properties are set in IB.  It makes extensive use of User Defined Runtime Attributes.