Use floats instead of ints for expression comparison

Summary:
Previously, we only chose integer values for our variables
when testing whether two expressions are equivalent.
This causes problems on periodic functions that may look
simpler for integer values than they actually are (i.e.
sin and cosine, among others).

This diff adds a `randomFloat` function and uses it to
pick values instead of _.random (which always returns
integers), and then compares the expression at those
values.

Test Plan:
 * added new tests to test.html to check for some previous
   problem cases (around trig functions)

Reviewers: justinjaffray, alex

Reviewed By: alex

Differential Revision: http://phabricator.khanacademy.org/D8490