log error when >1 incomplete exam record found (#1729)

Would be nice to know if this ever happens in production.
interactive autograder tests (fix #1726)
interactive ag test: bogus grader endpoint for jenkins (#1726)
interactive autograder tests: get selenium working again (#1726)

Globally overriding urllib2 with our URL handler causes problems for
anything using urllib2 afterward.  Selenium gets bit by this, but could
have been anything.
interactive test: add better docstring (#1726)
revert change to update_score, avoid conflict w/ jbau's refactoring (#1726)
assertEquals was what I wanted, not assertTrue (#1726)

per @jbau review.  I was mistakenly doing assertTrue, which made it not
much of a test at all.  by switching to assertEquals it turns out I
should just be treating the feedback as opaque, which is what I'm doing
now, both cleaner and easier to maintain.  It's never interpreted
anyway...
Merge branch 'master' of github.com:Stanford-Online/class2go into sef-1726-interactive-autograder-tests

Conflicts:
	main/courses/exams/views.py
add useful logline back into get_or_update_incomplete... (#1726)
Add two tests: grader functions, points possible (#1726)
logger.error instead of logging.error (#1726)
Merge pull request #1767 from Stanford-Online/sef-1726-interactive-autograder-tests

interactive autograder tests (#1726)