better TIME / DATE / DATETIME fields handling

* The output format of TIME fields changed from "HH:MM:SS" to
"[-][H]HH:MM:SS[.fractal]". See
http://dev.mysql.com/doc/refman/5.6/en/time.html for details
* NULL values in DATE, NEWDATE, TIME, TIMESTAMP and DATETIME fields are
now handeled correctly
* DATETIME and TIMESTAMP flieds now may contain a trailing fractional
seconds part in up to microseconds (6 digits). See
http://dev.mysql.com/doc/refman/5.6/en/datetime.html for details