Improved randomize module: DO NOT MERGE - sample code for @shnayder

randomize module which allows staff to select which specific child module to view.
Includes randomize_control.html template.  Uses AJAX for rendering children.

Does not include the code changes which would make randomize more robust, by tracking children based on location instead of index into list of children.  Using an index is not a good idea, because course staff editing the content may change the length and order of elements.  Additional error checking should be added to make sure the randomly chosen child still exists.

This version of the module was used at MIT through the Fall of 2013.  It is out-of-date with respect to changes which have happened to the xmodule semantics.

@shnayder