Create press state for categorizer

Summary:
{F58527,size=full}

(I tried a couple things for the press color--still don't love this, but let me know @marcos if you have an idea for making it better)

Test Plan: Go to a problem (like trigonometric-functions-and-side-ratios-in-right-triangles) and tap circles. So responsive!

Reviewers: marcos, mgp

Reviewed By: mgp

Subscribers: aria

Differential Revision: http://phabricator.khanacademy.org/D13273