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.
Next is used a lot internally and cannot really be changed to "continue" since it wouldn't make sense. Since the user don't see much about that wording... I would let things as they are for now.