Avoid creating unused cron events

Create a cron event for each likely cron schedule.  The likely cron
schedules are the default WordPress cron intervals (hourly, twicedaily
and daily are defined in wordpress 3.5.1) and any cron schedules added
by our plugin or it's close relatives.  We recognize these by checking
if the schedule name is prefixed by 'wpsc_'.
Merge branch 'cron-event-fix' of https://github.com/JeffSparkleGear/WP-e-Commerce into cron-event-fix