Expose a function for sub-millisecond timestamps

[andrewbenton]

The current Date.now() function returns an integer indicating the number of milliseconds since the unix epoch. Only having millisecond precision is an issue because multiple events can occur within the same millisecond, which makes event ordering uncertain when submitting events upstream. Most platforms expose microsecond or nanosecond time sampling through the C / C++ API. While this number can't fit inside the mantissa of a javascript number (53 bit limit), it could be provided as a non-standard representation and be very useful.