Improvements and fixes to TimeSpanUnits and TimeSpanSerializer:

1. Add a new Microseconds value to the TimeSpanUnits enum
2. Fix multipliers for Nanoseconds in TimeSpanSerializer
3. Fix precision issues in TimeSpanSerializer (see note)

Note: the precision issues in TimeSpanSerializer are courtesy of Microsoft implementing the FromMilliseconds and all other FromXyz methods to limit the precision of the value to milliseconds instead of ticks. Not sure why they did that... The work around is to always use FromTicks instead.