PropCheckBox: move file and use nicer requires

Test Plan:
 * visit test.html
 * add a transformer
 * check and uncheck "graded"
    * verify that the toJSON graded prop changes
 * uncheck all the transformation tools and make sure they disappear

Reviewers: alex

Reviewed By: alex

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D5844