General cleanUp of app.js

- clean up some comments
- clarify others
- replace boy with cat girl
Add gameover state

- Once player dies the game stops and displays game over
- implement click handler that resets and resumes game
- add onscreen text that notifies player
Add displayOnscreenText function & comments

- displays canvas text
Add clean up for Exit class .reset method

- Exit obj renders key that is outside the playing area
  without the cleanup it remains onscreen
Add delay on level end

- Once the player gets the key
  there will be a 2 sec pause before the reset

- Disable player input during that time