trivial but useful script to help be debug Hive pipeline by looking at file sizes in data partitions

Summary:
helps me very quickly spot when, say, a day or problem logs is missing, or where the Incr/P process
went awry fro any given type

Test Plan:
ran it, got the following output

jace-> python tools/s3_du.py s3://ka-mapreduce/entity_store/UserDataP 2013-09-01 2013-09-04
11121328249 s3://ka-mapreduce/entity_store/UserDataP/dt=2013-09-01

11208060648 s3://ka-mapreduce/entity_store/UserDataP/dt=2013-09-02

11275057415 s3://ka-mapreduce/entity_store/UserDataP/dt=2013-09-03

Reviewers: eliana

Reviewed By: eliana

Differential Revision: http://phabricator.khanacademy.org/D3947