Support ISO-8601 representation of intervals

[GoogleCodeExporter]

For some reason our JSON representation of an Interval is something like this:

    { Start: "2013-07-26T16:45:20.1234567Z",
      End: "2013-07-26T17:45:20.1234567Z" } 

When it could be just a single value:

   "2013-07-26T16:45:20.1234567Z/2013-07-26T17:45:20.1234567Z"

That's a natural extension of ISO-8601 to cover subseconds.

This should probably be our ToString representation too (which is currently 
explicitly not defined).

Original issue reported on code.google.com by jonathan.skeet on 21 Feb 2014 at 9:22