Add float support to arithmetic operators

Summary:
Filled in the TODO for checking ArithmeticOperator types.  Handle mixed
int/float arithmetic.  Add float literals to the compiler.

(Also fix existing lint errors in these files.)

Test Plan:
add tests for float-only and mixed int/float arithmetic
`<webappdir>/tools/runtests.py`

Reviewers: alan

Reviewed By: alan

Subscribers: jace

Differential Revision: http://phabricator.khanacademy.org/D11155