Update mlw_quiz_admin.php
Minor Design/Database Changes

Made quiz table display 20 quizzes at a time. I converted more of the wpdb->query to updates.  Put the MathJax js script inside a wp_enqueue_script call.
Replaced query with wpdb->insert

Finished converting from wpdb->query to the better wpdb->insert and wpdb->update functions.
Merge pull request #49 from fpcorso/feature/quizzes_design_changes

Feature/quizzes design changes