Functions!

Summary:
Added full support for logarithms.
Added basic support for trig functions.
Added full support for abs() and sqrt().
Added basic support for |x| notation.
Added full support for variable subscripts.

General code cleanup:
Removed a lot of brackets and parentheses.
Renamed distribute() to expand().
Moved asTex() to within ExpressionTools.

TODO:
Let content creators define which variables are functions.

Test Plan: src/expressiontools/test.html

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D3153