(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.
Would need to be shared too.

Btw, it would make sense to create an `xblock-toolbox` repo for the generic functions/files/mixins that we want to reuse across all repos. If that works for you I'll create the repo.