Refactored get_current_term_exams to group sections.

Closes issue #99 by fixing the problem when courses are imported from OpenData but before they are saved to Mongo, and closes PR #96. 

Sections for each subject are ordered before being saved, so that it's easier to find any one section on the table.

Tested by running:

`PYTHONPATH=.. python data/crawler.py opendata_exam_schedule`  and then
`PYTHONPATH=.. python data/processor.py exams`

to ensure that the exams are grouped together appropriately.