Added in a retry for checking the class of the video
Made a new command css_has_class that will safely check the class and get around stale elements
Refactored LMS so all potential stale element exceptions are avoided
CMS acceptance tests now should not get stale element exception errors
Fixed structure of css_text and css_value
Reworded css_text to use css_find
Revert "Reworded css_text to use css_find"

This reverts commit a7de337fe2d218d6cb9c2f175c6ea71f5ddcb6da.
Changed a world.browser.find_by_css to world.css_find
Made a retry_on_exception world function that will retry the given function
Refactored the ui tests to use retry_on_exception
Refactored problem_setup to use retry_on_exception
Refactored recently merged in tests
Merge pull request #333 from edx/jonahstanley/fix-flakey-video-test

Jonahstanley/fix flakey video test