XBlock

This is a very basic XBlock integration. It will support nicely editing in the Studio and using in LMS, but every time starting from zero, and the answers are leaked client side.

I have some work on moving to server-side, but I decided to rollback for the sake of delivering today.

Following is a TODO list

 - [ ] weight / has_score / score event
 - [ ] saved, server side item state
 - [ ] server side checking and feedback
 - [ ] ability to edit in the builder
 - [ ] full HTML editor for the question text
 - [ ] serve the triangle img
 - [ ] vendor remaining jQuery UI css
 - [x] cancel button

I also have a couple of thoughts on how dependencies are handled, but I'm too tired to write them down.

@antoviaque 
This loads the JS/CSS directly on the page in the LMS, preventing caching. This is less of an issue on apros given the way jquery-xblock loads the content, but `add_javascript_url()` and `add_css_url()` should still be preferred.