Improved ISO8601 support

Improved the ISO8601 format to use the round-trip format to supports milliseconds, timezones and UTC time.

Added three unit tests to make sure the serialization has the required result for all different kinds of datetimes (Local, UTC and Unspecficied) in both formats, ISO8601 and WCF.

Also made the ISO8601DateFormat  locally overridable on the DefaultJsonSerializer, just like the RetainCasing JSON setting.