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.
Not for now, but to remember to consider it for the next time - since we are working with XML this type of validation might be done through an XSD. Cf the conversations happening on the mailing list about having each XBlock define its own: https://groups.google.com/forum/#!searchin/edx-code/xml/edx-code/zJRzz8KCQ30/d0HyGLpu_GkJ