Microseconds for DATETIME values within range of the Time class

Per suggestion of @miyagawa's issue #324 add support for microseconds in DATETIME fields, available starting in MySQL 5.6.4. The extra scanf fields will be ignored if not matched.

Cleans up use of unsigned ints in the date/time scanfs.
To complete the thread, microseconds in TIME added in cb42b7367b056a2cd403dfebc44c1aba9783d435