CLC-2184 Give Canvas proxy its own sense of multiple terms

JIRA: https://jira.media.berkeley.edu/jira/browse/CLC-2184

I separated CanvasProxy's sense of multiple terms from the SakaiProxy's. I then ran a test import (bundle exec rake canvas:full_refresh) on my localhost against a local instance of Canvas that has an artificial Fall 2013 term with 1 course and a single section. The import worked successfully and refreshed test user Tammi (300939) and her enrollment in the Fall 2013 course.