Rational numbers! Bug and parsing fixes!

Summary:
Also:
Fixing linting and expression tests.

Merge branch 'master' into expressions

Rational numbers! Bug and parsing fixes!

Now with tests for new behavior.
Also:
Fixing linting and expression tests.

Merge branch 'master' into expressions

Merge branch 'master' into expressions

Test Plan:
src/expressiontools/test.html

src/expressiontools/test.html

Reviewers: alpert, eater

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D2610