Full equation support + many bug fixes + test refactoring.

Summary:
Full support for comparing and simplifying equations.
Inequalities are handled appropriately (can't multiply/divide by factors
you don't know the sign of).
Fixed how distribute() works on rational expressions (numerators and
denominators are now distributed separately).
Refactored test suite so that relevant functions are near usage, and
replaced strict print() syntax with another parse() where possible.

Test Plan: src/expressiontools/test.html (now more coherent and easier to read)

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D2925