use read-only submodules by default

The read-write submodules are intended for the project owner
and the typical user would face errors about host key verification
failed.
rename buttons/leds to ABCD

The Simon kit does not specify the led layout so each device will
have a different layout which makes the code very confusing.
allow player to setup colors for their device
update readme for lilypad, baud rate and app build instructions
add Google Play Store assets