Add log_10() function to calculator

Test Plan:
Loaded http://exercises.ka.local/test and watched tests pass
Loaded http://exercises.ka.local/exercises/law_of_cosines.html
Calculator looked reasonable.
Clicking "log" button inserted "log("
Entered "log(10)" and got 1
Entered "log(1000)" and got 3
Entered "log(0)" and got an error
Entered "log(0.001)" and got -3
Entered "log(e)" and got something irrational looking (0.434294482)
Entered "ln(e)" and got 1

Reviewers: charlie, joel

Reviewed By: joel

Subscribers: joel, justin

Maniphest Tasks: T2681

Differential Revision: http://phabricator.khanacademy.org/D10486