adds sequel 'models' plugin

- assumes postgres, and enables support for postgres specific features,
  including hstore, array, json, streaming responses, and postgis
 - Installs sequel_pg, for better performance
- Includes support for migrations, e.g. `rake db:migrate`
  - Migrations are stored in: `./db/migrations`
  - Uses sequel integer based migrations
- Check the generated `./config/sequel.rb` for database related
  environment variables to configure sequel (max connections, connect
  timeout, log level, etc)
Bump to 0.2.0