OEC-460, Primary OEC script: Generate courses.csv per dept with naming _...

https://jira.ets.berkeley.edu/jira/browse/OEC-460

Rake task oec:courses generates XXX_courses.csv per department. This required an overhaul of courses_spec.rb, etc. Rake task oec:students now iterates through all XXX_courses.csv files.  
@johncrossman do we ever have a department with 2 spaces in its name? Otherwise we should probably use `gsub`