Query to find the most active coaches, based on student activity, for a given start and end date

Summary:
This query looks at some existing data sets to compute student activity for the start and end date.  Then it sums these numbers up for each coach and looks up their geographical information from the teacher_country table.  The final result looks something like this:  https://docs.google.com/a/khanacademy.org/spreadsheet/ccc?key=0AhauPmyK0zDEdExnS2xqNkhqQjFSQlZ0OWVxY3I3bFE#gid=0

I think there is a problem where some of the coaches are being dropped, since the final table only has ~1900 rows.  I suspect that the teacher_country table is not as full as I thought it was.  More debugging is required after I get some feedback from Maureen.

Test Plan: Copy+paste stuff into hive, see if it works.

Reviewers: jace, robert

Differential Revision: http://phabricator.khanacademy.org/D4193