Tabbing through captions works except for persistence of outline after mouse click
Tabbing through captions is completely functional.
Adressed pull request comments
Removed unnecessary event handler
Added comments
Moved css from js file to relevant scss file.
Added tests for tabbing through captions
Bug fixes, mainly in tests
Replaced mouseenter and mouseleave event handlers by mouseover and mouseout. The former were failing Jasmine tests. Since the captions are li elements without descendants, they are identical.