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.
Eventually this workaround won't be required when native XBlock children are supported by the LMS - converting this to use real XBlocks will be the real cleanup here. I'm fine leaving as is, but definitely include the full comment from Braden above.