OEC-358, add secondary section cross-listings query

'rake oec:courses' output was missing secondary cross-listings external to the OEC dept set. This commit adds 'get_secondary_cross_listings' query and reusable SQL fragment.
The test data (courses.csv) is used as (1) mock query results and (2) sample csv output. I converted keys to lowercase such that all column data is picked up. This "fixed" a problem whereby 'cross_listed_name' was empty. Now it returns values like "STATS SOCI SCI (72198, 87690)" which, in turn, exercises the get_by_cnn(72198, 87690) query. Bottom line, I had to revise courses_spec to expect get_all_courses with '54432, 87672', etc.