Added JSON serialization support for DateTimeOffsets

Following on from PR https://github.com/NancyFx/Nancy/pull/1578 (which became a bit of mess) I've made a fresh fork on a proper branch, which hopefully will pass the tests on Mono as well as Windows. Fingers crossed...

From original PR:

I ran into problems trying to use DateTimeOffset types in my resources - because the default Serializer writes these as basic JSON objects rather than proper date strings, they don't serialize and deserialize properly (similar to this old issue with - Timespans https://github.com/NancyFx/Nancy/issues/336). Rather than creating a converter it seemed more sensible to serialize DateTimeOffsets in line with the serialization of DateTimes.