OEC 476, rake task to diff dept spreadsheet against current campus data

https://jira.ets.berkeley.edu/jira/browse/OEC-478 and sub-tasks

Rake task: 
> rake oec:diff dept_name=INTEGBI 

Generates a report highlighting diffs between dept provided data and most recent campus db. Report is written to data/2015-B/csv/work (per Justin reqts) and has format:

> KEY | COLUMN | CAMPUS_DB | DEPT_FILE
> 2015-B-42271-172364 | FULL_NAME | Alan Shabel | Alan S. Shabel
> 2015-B-42325-1077796 | EMAIL_ADDRESS | pkrigas@gmail.com | pkrigas@gmail.org