Dot plot now works on touch devices (and mobile app in particular)

Summary:
The dot plot was using some strange touch handling mechanisms, most notably an onMouseOver
call that wasn't getting triggered on touch devices. I refactored the touch handling code to be
kinder to touch devices, which fixes several exercises on mobile.

Test Plan:
- Tested in an emulator and on the mobile app (try the "Creating Line Plots 2" exercise).
- Tested in Chrome to verify that nothing broke on browser.

Reviewers: jack

Reviewed By: jack

Differential Revision: http://phabricator.khanacademy.org/D11029