Add class size histogram!

Summary: As in the title. We will be able to validate > 10 class size definition of teacher. That's a first step towards it. Next will be ui. But there will be more coming

Test Plan: run map_reduce/hive/class_size_histogram.q

Reviewers: jace

Reviewed By: jace

Differential Revision: http://phabricator.khanacademy.org/D3033