Make basic non-perfect cube root exercise
Add answer type for cube roots
Show factor tree in two hints regardless of size
Merge branch 'cube_2' of https://github.com/petercollingridge/khan-exercises
Make cube root a little prettier and don't accept blank answers for cube or square root answers

Test plan: Try answering some simplifying_radicals and cube_roots_2 problems.

Auditors: peter, alpert