More expression fixes!

Summary:
No more strange subtraction issues in rational expressions.
3/4x and 3x/4 are now parsed equally but render as you would expect.
3/(4x) is now equally as simplified as 3/4 x^-1.
Rendering is more faithful to user input now.

Includes nits from previous fix.

Test Plan: src/expressiontools/test.html

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D3297