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.
Yep, that's what jquery.xblock.js is used for. However, it has the benefit to include everything needed to load a xblock or child with all its dependencies. I like the idea to reuse something that we already done and it's avoid me to do that manually. The main reason to use it is that step are loaded one at time, so jquery-xblock takes care to load all css/css_url/js/js_urls. The only problem for now is that the xblock cannot really communicated with its backend endpoint (ie. to publish an event). But we would have had the same issue display everything manually.