Fix screen capture for Mac.

[octalmage]

Resolves #188.

For higher density screens (Macs), this PR means the resulting screen capture could be larger than the area requested. You can work around this by dividing the image size by the requested size. For example:

var size = 10;
var img = robot.screen.capture(0, 0, size, size);
// Support for higher density screens.
var multi = img.width / size;

var color = img.colorAt(2*multi, 3*multi);

This might be built into RobotJS at a later point.