Make the number answer type more flexible

Summary:
Add the ability to have inexact answers as well as different number
forms than before.

Test Plan:
Select the number answer type, enable inexact answers, make sure an
answer within the error tolerance is accepted. Select a different answer type
(like mixed numbers), and make sure it accepts numbers with that form. Also,
type "3 pi" into the correct answer box, and make sure it expands to
"9.42477796076938".

Reviewers: alpert

Reviewed By: alpert

CC: eater

Differential Revision: http://phabricator.khanacademy.org/D2641