Initial implementation of the adventure xblock

Initial implementation of the adventure xblock, build with Backbone + Marionette.

It currently only read the raw xml config.... but this will be modified to use lightchildren. To make this testable on the client side, we can only mock the function that fetch the data in the controller.
@aboudreault This is just a question to help me learn: as far as I understand, jquery-xblock was created to load XBlocks across domains, for client frontend websites etc. - which is not something that would normally happen on an edX instance. But in this case, we're just loading an XBlock and displaying it as a child of another XBlock on the same site, which seems like a fairly basic/fundamental thing to do. Is this not something that the runtime or the XBlock library or something standard like that should be able to do already without needing a third-party jQuery plugin?