Increase compatibility with postgres time type.

Allow 24:00:00 for time value when parsing, as this is a valid value for PostgreSQL. I got errors when I tried to use Tod with previous entries, that were already in DB (the value of 24:00:00 is created by using end_of_day). It would be hard to allow value of 24:00:00 internally, thus it is casted as 23:59:59.

It could be useful to also allow decimal seconds, but I currently use time(0) as column type to avoid issues with this.

More information:
http://www.postgresql.org/docs/9.3/static/datatype-datetime.html