(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.
Don't alter the history - if a student revisits a previous step, it adds to the list of visited steps so that his complete path can be tracked.

Also, it should be controlled by `<step back="[step-name]">` - it doesn't redirect to the *previous* step, but rather to an arbitrary step, identified by name.