(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.
Currently, yes... the adventure id (url_name) is used for the as the main adventure id and used on client side too for the element id and marionette communication channel id. So removing it would need something else I guess. Could be a random aphanumber generated, so url_name could be remove completely.  Will take a note about this.