Added exams complete counter to landing page for logged in users.
Removed multiplication symbol "x" from count display.
Hide count label when count is 0.
Add check for user account associated with user_id in exam object in case user deletes their account.
Merge pull request #2 from glenn0/add_exam_counter

Add exam counter
Refactor JavaScript to toggle hidden state of answers
Add counter to display number of completed quizzes on the main show page