Expose image stabilization for AVCaptureStillImageOutput

[eliburke]

Logan, thanks for your work on this wrapper. I've got a suggestion for an improvement. I didn't think a PR was necessary as it is pretty straightforward:

In the header file:
@Property BOOL isStabilizedMode;

In setup after the AVCaptureStillImageOutput object is created:
if (isStabilizedMode && _stillImageOutput.isStillImageStabilizationSupported) {
_stillImageOutput.automaticallyEnablesStillImageStabilizationWhenAvailable = YES;
}

This is iOS7+, so if you support older code bases it should be wrapped in a respondsToSelector