PostgreSQL index options

PostgreSQL allows to specify some options for columns in index. For now, Sequel supports only one such option (`:opclass`) but it's doing it in such way that if one uses composite index, Sequel will stupidly assign one opclass for each field. This PR fixes this behaviour.

Also, Postgresql >= 8.3 supports `NULLS FIRST | NULLS LAST` and `ASC | DESC` settings. This PR adds possibility to rule them through `:order` and `:nulls` options accordingly.