Exposing head orientation

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This change does two things to expose the head orientation parameters from the CBDViewController. First, it adds the eulerAngles() method to export results from the headTransform.eulerAngles() method. This makes it more convenient to pass orientation tracking information to Objective-C clients. Second, this change adds offsetPan and offsetTilt properties, which are used to offset the view direction when vrModeEnabled is off. This makes it much easier for clients to add manual pan and tilt adjustment for devices that are not in a cardboard viewer.