Support \pi in 'pi' answer-type

Summary:
On mobile, the MathWidget returns \pi when you write pi, and it makes more sense
to address that fix at the source (i.e., here) than to add more scary TeX parsing elsewhere.
I also think that this is a good thing to support on the web, too, where some people would
genuinely opt for this approach in the absence of any other instruction. (Note: all existing
behavior should be maintained, if I did my regex right.)

Test Plan:
- Run khan-exercises.
- Go to "circles_and_arcs.html".
- Use 'pi' for 'pi' (to verify that existing behavior is maintained).
- Use '\pi' for 'pi' (to verify that new behavior works as intended).

Reviewers: eater

Reviewed By: eater

Subscribers: andy, aria

Differential Revision: http://phabricator.khanacademy.org/D12050