AXRDocument should provide methods to access loaded URLs

[jakepetroules]

Currently an AXRDocument is composed of an XML file (which may be in-memory) as well as other HSS stylesheet files, and in future, JavaScript files.

AXRDocument needs to provide methods for users to access the AXRController and URL of its main file, as well as all "sub-files" that are part of that document.

For example:

• AXRDocument::controller() // controller for the main XML, may or may not be in-memory
• AXRDocument::url() // URL for the main XML, may be a null URL for an in-memory file
• AXRDocument::stylesheetControllers() // list of controllers for associated HSS stylesheets
• AXRDocument::scriptControllers() // list of controllers for associated JS scripts

AXRController should probably keep track of the URL (then we can iterate the return value of stylesheetControllers() and easily get all the URLs).

Finally, we'll need events that fire when controllers are added/removed, etc.

See also: axr/browser#11