Add state for 'active' (pressed) elements

Summary: This is a first pass at press states for radio buttons in Perseus--once we've settled on design, I'll make it work with khan-exercises too.

Test Plan:
Load a Perseus exercise with radio buttons; make sure the text is legible and the press state feels more responsive.

pre:
{F57923,size=full}

post:
{F57922,size=full}

Reviewers: marcos, mgp

Reviewed By: mgp

Subscribers: aria

Differential Revision: http://phabricator.khanacademy.org/D13223