SQL schema dump format support

Hi, I'm using database-side programming a lot and the ruby dump format is nigh useless. 

I've implemented sql dump format; the usage is the same as with active_record (just put `config.sequel.schema_format = :sql` into your app environment).

Currently the feature is only implemented for Postgres backend, but it's trivial to expand that to other dbms (just implement SequelRails::Storage::*#_dump and #_load).