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.
I also like the approach of not having a predetermined name for the first step, it's true that it's unecessary - we can definitely remove this requirements from the specs.

In any case, we should at the minimum address the inconsistency noted by Braden, between using both `self.steps[0].name` and `"first"`.