Query to compute total number of students and teachers   using heuristic to define teacher.

Summary:
Add student_coach_count to jobflow config

Add student_coach_count to hive init

Rename coach to teacher where appropriate

coach -> teachers rename

General idea is that we need to generate partial sums.
They way it is usually done is through cross product with selection. Though, in
hive case it would be better to write UDAF (only in java) for that.

Would be open for some suggestions about simplifying and optimizing it, however, I
think that's good for a first attempt. Especially due to the fact that it's independent
of the time range.

Test Plan:
Execute the queries with ka_hive_init.q

Check that student_coach_count.q is executed daily

check that student_coach_count is available after hive startup

None, rename

Reviewers: jace

Reviewed By: jace

Differential Revision: http://phabricator.khanacademy.org/D2309