Added propTypes to Measurer and Dropdown

Summary:
Added some propTypes and refactored Dropdown so that the widget
doesn't know which choice is correct (instead grading itself via the rubric).

Reviewers: jack

Differential Revision: http://phabricator.khanacademy.org/D9894
Added propTypes to Measurer and Dropdown

Summary:
Added some propTypes and refactored Dropdown so that the widget
doesn't know which choice is correct (instead grading itself via the rubric).

Test Plan:
- Run Perseus.
- Load this [Dropdown and Measurer](http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22%5B%5B%E2%98%83%20dropdown%201%5D%5D%5Cn%5Cn%5B%5B%E2%98%83%20measurer%201%5D%5D%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%22dropdown%201%22%3A%7B%22type%22%3A%22dropdown%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22choices%22%3A%5B%7B%22content%22%3A%22ABC%22%2C%22correct%22%3Afalse%7D%2C%7B%22content%22%3A%22DEF%22%2C%22correct%22%3Atrue%7D%5D%2C%22placeholder%22%3A%22Letters%22%7D%7D%2C%22measurer%201%22%3A%7B%22type%22%3A%22measurer%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22box%22%3A%5B480%2C480%5D%2C%22imageUrl%22%3Anull%2C%22imageTop%22%3A0%2C%22imageLeft%22%3A0%2C%22showProtractor%22%3Atrue%2C%22showRuler%22%3Afalse%2C%22rulerLabel%22%3A%22%22%2C%22rulerTicks%22%3A10%2C%22rulerPixels%22%3A40%2C%22rulerLength%22%3A10%7D%7D%7D%7D%2C%22answerArea%22%3A%7B%22type%22%3A%22multiple%22%2C%22options%22%3A%7B%22content%22%3A%22%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%22calculator%22%3Afalse%7D%2C%22hints%22%3A%5B%5D%7D). Confirm that there are no console warnings RE prop types.
- Confirm that the Dropdown grades correctly as well.

Reviewers: jack, eater

Reviewed By: eater

Subscribers: eater

Differential Revision: http://phabricator.khanacademy.org/D9894