Jrbl/xblock

First pass at XBlock for composition
    
    * Implement a basic XBlock for composing. Allows the user to enter text
      and see what they entered.
    * Dirty shell script to install the XBlock demos into this working tree
      so that we can use the workbench for early prototyping. This should
      not be used any more once we integrate into LMS, obviously.

@ormsbee @stephensanchez bump