Delete unused code
Use explicit parsers & builders for UTCTime and Day

- Make Day use the Database.SQLite.Simple.Time module in FromField
- Use Blaze builders for converting UTCTime & Day types to strings

Conveniently this also gets rid of the old-locale dependency in the
cabal file.
Add tests for parsing & building UTCTime and Day strings

Includes timezone offset tests which we didn't have before.