Supports inserting times at microsecond resolution

Postgres supports timestamps and times with up
to microsecond resolution:
http://www.postgresql.org/docs/9.1/static/datatype-datetime.html

pgsql already supports reading times with sub-second
resolution, and this commit adds support for inserting
times with up to microsecond resolution.
Add test for microsecond time inserts
Merge pull request #18 from sebastian/master

Support inserting times at microsecond resolution