Fixes to issues reported on "Orders of Magnitude" Perseus exercise.

Summary:
Anything that expression renders, it should accept.
- Accept \\cdot as input.
Most common error is trying to use "x" for multiplication.
- Be more explicit in examples about using * for numbers.
- Remove variable "x" from multiplication example.
Input=number does clunky validation, reformats answers, and blocks fractions.
- Remove it entirely.
Clicking on multiple choice labels no longer selects the adjacent radio input.
- Wrap radio inputs in labels, like it was before.

Test Plan:
    Expression tests pass.
    View new examples at testrender.html.
    Try items on iPhone.
    Click on multiplice choice labels.

Reviewers: alpert, jakesandlund

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D2691