Snes support

[mariaS210]

Since we started this implementation at EuroPython, here it is in a functional state.

Naming: gamepad/controller is joystick everywhere (sometimes shortened to joy). That could be changed, but since it was called like this in pygame, I thought to keep the naming.

Configuration: There are 2 .json files that would hold gamepad mappings - to keyboard and to specific gamepad button events (e.g X on snes has different mapping than X on xbox). A following improvement would be to add a configuration tool, with an image of gamepad, like you told us at the sprints - that would change the joysticks.json.
In regard to mapping to keyboard, the first gamepad has arrow keys + space, which allows plug-n-play for pgzero's examples: tetra, snake, pong, flappybird, asteroids, lander (a bit awkward here).

There are some unit tests checking the basic keyboard and joystick events.