Add to-sql-time

I couldn't find "to-sql-time" despite java.sql.Timestamp getting imported. I'd very much like to not having this floating around in my models.clj in future, so here you go.
to and from sql-time implemented. tests for to and from added. Added mention in README.
Adds MySQL time format to default parsers.
API cleanup

provisional API experiment

formatting, in-milli

yeah plural is better

to-epoch for clj-time

got rid of warnings, deprecated in-msecs

ordering