Added Protocol to grab Keyword Context for Widgets

Created the URXKeywordContext protocol which provides an interface for
getting keywords from an object. The URXWidgets framework also provides
implementations of this protocol for most standard UI elements that can
provide a standard text output (UILabel, UITextView, UITextInput,
UINavigationItem, UIButton, etc.) as well as an outlet on the
URXListenButtonView widget which lets you connect UI elements to it in
interface builder to grab the context from.