Big refactor in preparation of introducing alternative document parsers.
The CSS, XML, Image, etc Resource objects were unified in scope of what they contain. The vaguely defined InputSource has been removed from the Resource interface and now we are free to choose any object to represent the resource.
It is now expected for the UserAgent to parse the resource input of all objects it returns, including style sheets (represented by CSSResource).
An AbstractUserAgent class was introduced to encapsulate the common (and basic) behaviour for delegated resource loading, URI resolution, creating input streams on resources and parsing style sheets.