Fix for timestamp precision in SQLAlchemy

This fix adds the PreciseTimestamp custom type to address how
MySQL < 5.6.4 truncates temporal columns to the second:
http://dev.mysql.com/doc/refman/5.6/en/fractional-seconds.html

Change-Id: I9d324d1cb6867cf99e15e2ecdc566dc5f8fef536
Closes-Bug: #1215676