Use input type=number for the integer and decimal number formats.

Summary:
Whitelist specific number formats, and use type=text for questions which accept
anything other than integer and decimal (e.g. pi).

Test Plan:
Tested a few exercises from Addition and Subtraction, Fractions, and Decimals
on Chrome on Mac OS X, Mobile Safari on iPad, and Chrome on Nexus 7.

Reviewers: tom

Reviewed By: tom

Differential Revision: http://phabricator.khanacademy.org/D1015