Catch - execute commands throwing errors

Without a proper catch, 'execute as_op' could allow players to remain as
operators!
Merge branch 'master' of https://github.com/aufdemrand/Denizen
Add weeks to durations

ALSO, all d@ notation. Not sure why this didn't already work
ALSO, notate dPlugin in the object type list
Catch nulls in the InventoryHolder property, fixes #625

Fairly sure the && on line 79 shouldn't be there... but @Morphan1 keeps
insisting that it's needed for something, so just catch nulls manually
for now.
Add element.round, correct math-exceptions, fixes #624

Also clean some meta
and rework round_up/round_down a bit to fit existing code
Also adjust inventoryHolder code a bit to prevent merge issues
Merge pull request #628 from mcmonkey4eva/master

Fixes and features and all that, pull requested!