Added joystick vibration support for most common controllers (resolves issue #159).

Added Joystick:setVibration(left, right), Joystick:getVibration, and Joystick:isVibrationSupported.
left and right are numbers between 0 and 1 which correspond to the strength of the gamepad's left and right rumble motors, if it has them.

src/modules/filesystem/physfs/Filesystem.cpp

@@ -31,7 +31,7 @@ namespace
 {
 	std::string getDriveRoot(const std::string &input)
 	{
-		for (int i = 0; i < input.size(); ++i)
+		for (size_t i = 0; i < input.size(); ++i)
 			if (input[i] == '/' || input[i] == '\\')
 				return input.substr(0, i+1);
 		// Something's horribly wrong