Make named_timezones extension usable by databases allowing timezone strings to be given to Database#timezone=

named_timezones was added before the database-specific timezone
support, and the string -> TZInfo::Timezone conversion was only
done in the Sequel module, not for Sequel::Database instances.
Refactor ::adapter_method

- somewhat simpler
- stricter values of opts[:database]; non-string arguments are
not accepted
Merge branch 'refactor-adapter-method' of git://github.com/eval/sequel