Changed button "Play Now" to "Play!"

"Play Now" is too long and sometimes gets wrapped on small resolutions.
Added displaying of currently logged in user
Added sign-out functionality.

If the user wants to switch accounts, he/she may now sign-out using a
sign-out button.
Some CSS design modifications and renames
Added loading screen

To provide a visual cue for the user, a loading screen will be shown while
the different games are being loaded in the background.