Refactoring and image output

* Version and reformat podspec
* Ignore AppCode files
* Refactor drawing into `CALayer` subclass closes #7
* Draw using `UIKit` instead of `CoreGraphics`
* Selectors rendering `UIImage`
* Icon view and layer share a protocol
* Icon view just forwards missing methods to its layer
* Padding proxies to inset to accommodate different axes
* Icon selector to retrieve by font name
* Integrate new features into sample app
* Adhere closely to style conventions