Change default close key to ` (vs ESC) due to pointer-lock. Closes GH-1

Even though using <escape> for closing the modal element would be a
logical choice, this key is already used by browsers for when the
user wants to explicitly exit pointerlock. Which wouldn't be a problem
in itself, but browsers (at least Chrome) don't allow applications
to automatically reenter pointerlock after exited with <escape>. Any
other key is fine, just not escape.

See http://www.chromium.org/developers/design-documents/mouse-lock

> Also, when an application exits (i.e. not due to the user pressing ESC,
> etc.) and reenters mouse lock there is no user gesture required and
> no exit instructions are shown. This provides for an improved user
> experience when an application frequently shifts in and out of mouse
> lock, such as a game with first person controls but a cursor used
> for inventory management.

So we have to use something else, for better or worse. I picked
backquote (`) since it is near the escape key (at least on my keyboard)
and is commonly used for gaming consoles. If your keyboard doesn't have
it, note you can still exit by clicking the main game canvas area.
preventDefault() when handling close key
0.1.1