Add TimeSpan datum converter

TimeSpan datum type converter should be added, serializing to a floating point number of seconds (eg. .TotalSeconds).  Also would be nice to support the time math operations supported by RethinkDB:

* time.add(number) -> time
* time.sub(time) -> number
* time.sub(number) -> time

These operations should be expressable in lambdas as both operator overloads and instance methods; both seem to be applicable.