Add `schema_owner_is()`.

Ref issue #40.
I kant spel.
Foreign key table must be visible if no schema specified.
Note fk_ok() fix.
Non-schema-qualified tables must be visible when testing constraints.
Non-schema-qualified tables must be visible when testing triggers.
Don't report a column as missing when its type is not visible.
Make sure tables are visible when testing FKs.
Remove format_type(); use pg_display_type(), instead.
Need to update `pgtap_version()`, too.
Avoid displaying a type schema twice.
Add missing doc header.
Note that one can include the schema for the type in the type arg to col_type_is() and domain_type_is().
Add tablespace_owner_is()`.

Ref #40.
Make sure tables are visible when checking indexes.

Also try to use parameters in the order in which they are passed.
Wordsmithing.
Add index_owner_is()`.

Ref #40.
Add language_owner_is()`.

Ref #40.
Concatenate less.
Add opclass_owner_is()`.

Ref #40.