Add capability to return the non-inverted button states

In some situations, such as checking for many simultaneous button presses,
it could be helpful to work with the "raw" output from the gamepad.
Pass false as an argument to `uint16_t getButtons(boolean return_inverted)`
to get the non-inverted output from the controller.
You don't have to pass any arguments to getButtons, the default is true
(return inverted output).
Add example sketch 1: SNESpaduinoPrintToSerial

This example shows how one can use the SNESpaduino library to get the
button states and parse them. Code explains itself through extensive
use of comments.
Add example sketch 2: SNESpaduinoNonInverted.ino

The sketch explains the use and possible benefits of the non-inverted
button states. Mainly: Checking for many simultaneous button presses
at once.