Add fractionVariable function to draw varibles with fractional coefficients nicely

Summary:
Convert first exercise to use fractions

Make simplification optional

Create second exercise

Change logic of the hints

Allow equation order to change within an exercise type

Add order to second exercise type

New exercise type

Create first decimal exercise

More exercise types

Test Plan: Try all the exercise types. Make sure the hints are correct.

Reviewers: eater

Reviewed By: eater

Differential Revision: http://phabricator.khanacademy.org/D9461