Merged SDL2 changes from bartbes/love-experiments/SDL2 into default.

- Added love.system module, including clipboard functions.

- Added custom hardware cursors (love.mouse.newCursor, love.mouse.setCursor.)

- Revamped love.joystick:
-- added Joystick objects and moved love.joystick functions which operated on individual joysticks to methods on the Joystick objects.
-- Added love.joystick.getJoystick and love.joystick.getJoystickCount.
-- Added events for when joysticks are connected and disconnected.
-- Added 'Gamepad' methods to Joystick objects: Joystick:isGamepad, Joystick:getGamepadAxis, and Joystick:isGamepadDown. They're a common abstraction for xbox controller-like joystick across operating systems.

- Added monitor choosing support and "fullscreen desktop" mode to love.window.

- Moved unicode text input events to their own callback function (love.textinput), removed the key repeat functions from love.keyboard and made the second argument of love.keypressed a boolean saying whether the keypress was a repeat.

- liblove now works with love.window and love.graphics in OS X

src/common/Reference.cpp

@@ -26,12 +26,14 @@ namespace love
 const char REFERENCE_TABLE_NAME[] = "love-references";
 
 Reference::Reference()
-	: L(0), idx(LUA_REFNIL)
+	: L(0)
+	, idx(LUA_REFNIL)
 {
 }
 
 Reference::Reference(lua_State *L)
-	: L(0), idx(LUA_REFNIL)
+	: L(L)
+	, idx(LUA_REFNIL)
 {
 	ref(L);
 }