Update TII plugin to support 2.4 fields

Hi Dan,

I did some work to make it use the 2.4 fields of the assign module. I have made it only use 2.4 assign fields, not 2.3 or 2.2 fields, so it would be best to have this in another stable 2.4 branch if you wanted to go that route.
One thing I could not work out was the new "extension" added to grades now in 2.4. How would this be reuploaded to TII when you grant extension? My "solution" is to remove the permission "grantextension" and make then adjust the "cutoffdate" which updates the assignment in TII next cron.

Thanks
Alex