Add iOS extensions to DeviceOrientationEvent externs

Only available in iOS 5.0 and later.
See https://developer.apple.com/library/safari/documentation/SafariDOMAdditions/Reference/DeviceOrientationEventClassRef/DeviceOrientationEvent/DeviceOrientationEvent.html
Merge pull request #391 from fredj/ios-externs

Add iOS extensions to DeviceOrientationEvent externs: webkitCompassAccuracy, webkitCompassHeading