CDRIVER-423 ISO-8601 string or $numberLong dates

Provide support for ISO-8601 strings and $numberLong dates.
Soo... if we support parsing leap seconds, do we want to actually convert them?  I.e. keep the fixed table of leap seconds that have existed in the past and update as new ones come out?

So  XXXX-12-31T23:59:60.000Z == XXXY-01-01T00:00:00.000Z? or not?  (and if not, is it an error to pass leap seconds that we don't already know about).  For what it's worth, the server just doesn't handle leap seconds (and I believe mongo export is what we're trying to be compatible with)