In game Pause menu - issue #73

Pressing the 'Escape' key now pauses the game and opens a pause menu.  The inputs by the player are blocked until they unpause the game.  Press either 'Escape' again or click resume to unpause.  

The other pause option is an 'exit' text that takes you back to the main menu screen.  The backend for player disconnects isn't fully implemented so it isn't a proper disconnect.

The screen itself is a copy/paste of the main menu art.

I also added a back button to the online lobby and smoothed out scene transitions.