Functions! (part 2/2)

Summary:
The missing pieces:
Content creators can now choose which variables are functions.
Expressions and equations with functions now take advantage of evaluation
as much as possible, and only then rely on simplification.

Also: This is the last edit to ExpressionTools within the Perseus repo.

Test Plan:
Making sure all tests pass on src/expressiontools/test.html
Making new items with and without function variables on local copy.
Saving, previewing, and modifying these items and confirming expected behavior.

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D3187