Fix issue #9: does not delete all data

Fixes #9 (closed)

The problem was api3 being used without removing the default 25 row limit, so only the first 25 jobs were fetched; deleted.

This PR uses api4 for MailingJob because well, why not, and because I couldn't be bothered to look up the syntax for using the api3 options array!

Still testing, but I think this will account for most of the weirdness.