Add Perseus API ClassNames for Radio widgets

Summary:
Adds three ClassNames to the perseus api:

    "perseus-radio-option",
    "perseus-radio-selected",
    "perseus-radio-option-content"

Test Plan:
Manually inspected a perseus radio widget and looked for
the appropriate classes

Reviewers: ben

Reviewed By: ben

Differential Revision: http://phabricator.khanacademy.org/D9956