NSDate deserialization from numbers.

My particular API provides dates as JSON numbers, since these parse an order of magnitude faster than a date string! There wasn't a built-in way to convert from JSON numbers (really `NSNumber`) to `NSDate`, so I made one.

The standard override on `<<<` assumes (justifiably, I hope) that the number represents a unix time stamp; that is, number of seconds since Jan 1, 1970 00:00 UTC. If the caller needs to customise how to create the `NSDate` from the `NSNumber`, they can pass in a function reference that does the conversion.