Does not create cron events for unknown schedules

Tests schedules to se if they are known before creating cron events.
Without the test cron events were being created for schedules inserted
by third-party plugins
removed .gitignore
Merge remote-tracking branch 'upstream/master' into only-known-cron-events-fix

Conflicts:
	wpsc-core/wpsc-functions.php
Merge pull request #485 from JeffPyeBrook/only-known-cron-events-fix

Do not create cron events for unknown schedules