(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.
A bit unfortunate to call it `render_js_template`, do we have a reason to keep it as that in the utils instead of naming it `render_script_template` for instance?

Or is it because it's a template used by JavaScript later?