Add support for milliseconds in json dates

- toJSON now includes milliseconds when converting dates, using
  the format "yyyy-MM-ddThh:mm:ss.fffZ". (3 decimal points on seconds
  seems standard)
- fromJSON supports dates with and without milliseconds
- updated DateTimeOffset conversion similarly

** The current master doesn't build on my machine, so I needed to develop with an altered fsproj, then cherry-pick just these bits of the change. Please confirm it builds and passes tests on your setup. :) ***