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.
Yes, it should definitely be something XBlock core would support - and it actually does already, in the workbench. But not on the LMS yet (long story short, the fact that the LMS has to support both XBlocks and the older plugin system, XModules, is creating complications that the LMS team hasn't entirely solved yet). In the meantime, we have to use workarounds, like LightChildren -- or here, using jquery-xblock.

Short of real XBlock children, I actually like the use of jquery-xblock here, it's true that it saves from a few additional dirty hacks to get the Ooyala XBlock working as a child.

Why is it that we can't communicate with the backend endpoint though? If we are able to query the view, the ajax calls should work too?