Added the two column Matcher widget.

Summary:
The left column can optionally be fixed, facilitating the "match up these words
and definitions" type of questions. Both columns can optionally be labeled.

Test Plan: Created questions using the Matcher on test.html and local webapp.

Reviewers: jack

Reviewed By: jack

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D6075