Adds IonImageView and IonButton that are usable in Interface builder

[maxcampolo]

I thought #20 was a good idea so I made two IBDesignable classes, IonImageView and IonButton. These classes can be used in storyboard to set images with IonIcons in interface builder.

Use by dragging a UIButton or UIImageView into IB as Set its class to IonButton or IonImageView respectively. This adds an Ion Icon Image inspectable property to interface builder as well as a Ion Icon Background Image property in the case of IonButton. This field takes a hex string, for example f450.

The classes are written in Swift which means the pod has to be used as a framework. If that's an issue they can be rewritten in Objective-C.