Edit event permission

In the other modules like node and book, there is a separate edit permission. But in the basicevent.module, there is no such equivalent. There is just an "edit event" permission and no "edit own event" permission. To make it similar to core node and book module, I think we should have this permission so one can edit events created by others without the need of the "administer node" permission.

The patch adds the needed permission and changes the update/delete check to see if this permission is set.