Add support for DateTimeOffset

Adding support for DateTimeOffset. 

Also note that this wiki page: https://github.com/facebook-csharp-sdk/simple-json/wiki/Date-and-Time-Formats seems to be out of date. simple-json does support `DateTime`. At least the Poco serialization strategy does. It assumes ISO8601 (good choice!).

You might want to add a virtual method that allows a strategy to choose a different date format later. :)