Optimization for mobile device

[yilunzhang]

Riddle seems to be optimized for desktop. On mobile device (e.g. iPhone, iPad), many features are not usable. For example, when user try to zoom or move screen with multiple fingers, they will draw lines instead of moving screen. Also, some features like remove a path seem to rely on keyboard. It would be great if Riddle can be optimized for mobile device so people can use it on their phone or tablet.