microsecond-bug-fix for datetime.datetime
microsecond-bug-fix

When the MySQL Datetime Fraction is less than 6, microseconds set
incorrectly. For example, you set the field, Datetime(3). Then this
library read the time `2013-11-07 10:27:35.705` as `2013-11-08
10:27:35.000705`.
Merge pull request #38 from olive-oasis/master

microsecond-bug-fix for datetime.datetime