Fix #11: Right click support.

When clicked, Button() and Input() return the respective mouse button
constant ('l', 'r', 'm') instead of just true.
When activated by keyboard, Button() and Input() return 'return'.
Otherwise Button() and Input() return false.

button.lua

@@ -72,6 +72,6 @@ return function(w)
 	core.registerDraw(id, w.draw or core.style.Button,
 		w.text, pos[1],pos[2], size[1],size[2])
 
-	return mouse.releasedOn(id) or (keyboard.key == 'return' and keyboard.hasFocus(id))
+	return mouse.releasedOn(id) or keyboard.pressedOn(id, 'return')
 end