Course complete indicator

Added green check mark to courses index page when student has completed all of the lessons for that course.

Pivotal Tracker story:  https://www.pivotaltracker.com/story/show/68161028

In order to test my code, I also added a helper in spec/support/selectors.rb that enables finding a link with a specific href.

Merged master branch into my branch and ran test suite again.  The first time through, one test failed:  spec/features/courses_lessons_spec.rb:532  

I ran the suite several more times and couldn't replicate the error.  I suspect it depends on the order the tests are run in.  The tests I added also mark lessons as complete or incomplete, which may throw off the later test depending on the order they're run in.