(WIP) 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.
We're trying to move away from having to edit the block before the default content is loaded (cf https://trello.com/c/yrIlzBoW/230-mckinsey-edx-mentoring-display-default-contents-before-the-first-edit ). Can you do it more like on the other XBlocks, and have the default XML loaded immediately when this is added in Studio? This is done like this in mentoring because we want the `url_name_with_default` value to be dynamic, and there used to not be a way to do that before the default could be a funtion. But I don't think we need it the adventure XBlock?