[BinaryJSON] Switch to NSDate for dates

[Danappelxx]

The only thing stopping me right now is that apple/swift-corelibs-foundation has not implemented all of the NSDate functionality and I don't want there to be inconsistencies across platforms.

Also, although this issue should probably be over at Danappelxx/BinaryJSON, I'm going to go ahead and make it here since I don't expect that it would get any activity there.