nowrap option for PostgreSQL

Encountered a use case where I needed to execute an existing trigger (`tsvector_update_trigger`) but this was not compatible with how the PostgreSQL triggers are declared, [as others have found](http://stackoverflow.com/questions/21147893/tsvector-update-trigger-cant-find-tsvector-column). This `nowrap` chainable method sets an option to indicate that the `raw_action` should be executed directly, rather than being wrapped in a `CREATE FUNCTION`.

Wrapping the `raw_action` only seems to happen to PostgreSQL, and it looks like this was to enable `security` and legacy support for `where`. I have added appropriate warnings for when these options are set in conjunction with `nowrap`.