Fix psql drop constraint issue on primary keys

[throughnothing]

This attempts to fix an issue when dropping a primary key in PostgreSQL.

Currently, the code handles foreign key dropping fine (by dropping the tablename_fieldname_fkey constraint), but ignores the case of primary keys (tablename_pkey constraint). This adds logic to handle that.

I have tested this manually and it fixes the issue I was having using DBIC::DeploymentHandler to generate a migration that changes the primary key column on a table. Additionally, all existing tests in SQL::Translator pass with this change.

I have added some very simple tests, based around the FOREIGN_KEY tests I found in t/47postgrs-producer.t, and I'd love some feedback on those as well. Are there other places I should be adding tests?

[ribasushi]

Finally applied as bc9b1c11f. Sorry for the delay.