MRIconView shapeLayer animation and appearance selectors.

[boherna]

Allows the shapeLayer of MRIconView to be animated. Adds the appearance selectors to MRIconView for setting the border and line widths, and also the animation duration.

[mrackwitz]

So much great stuff in one single commit! 🎉
Didn't want to leave you alone without feedback: so would you mind to integrate your proposed changes a slight bit more into the existing code base by changing indentation from tabs to whitespaces and perhaps reverting the removal of the newlines in the public interface?
Otherwise I love the idea to have animated transitions for the overlay by implementing the animation pseudo-protocol. I guess, that's what you use it for, right? I would love to make that a bit more obvious by exposing the implemented selectors startAnimating, stopAnimating, … in the public interface with some little words of documentation, which could be basically just copied over from MRActivityIndicatorView.