vector{Add|Subtract|Scale|Dot} functions in KhanUtil

Summary:
Tests for matrix functions

I am uncomfortable praying to unit-test dieties

Test Plan:
pray to the unit-test diety for unit tests?

manually tested in coffee/node

make test

Reviewers: alex, alpert

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D4772