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 #629 from Morphan1/master

I like trains
Merge pull request #628 from mcmonkey4eva/master

Fixes and features and all that, pull requested!
Add ability to give list of locations to PlaySounds. Partially addresses #626.
Allow playing effects on lists, fixes #626

Also correct playsound meta.
Also deleted pending playsoundcommand.java changes on the assumption
that Auf did it all properly
Merge pull request #630 from mcmonkey4eva/master

Pull request for half-commits
Update selected_npc tags to new Citizens version

> Players now uses new UUID system
> Server now doesn't throw error on null
Merge pull request #631 from mcmonkey4eva/master

Pull request for fixing things
Don't crash on improper ~walk usage

InstantQueues, foreach/repeat, null NPCs, ... so many ways it can go
wrong!
Rework instant injections

They now manually fire instantly, rather than relying on the revolve
code to handle it. This will dodge issues related to if/else leveling.
Also, the algorithm for instant code was reworked to be less likely to
conflict with future injection code.
Apply some auto fixes

Also, fix inventory.title
Merge branch 'master' of https://github.com/mcmonkey4eva/Denizen
Add - inventory close

This simply closes any inventory the player has open.
Allow tags in inventory script titles and definitions

This is mostly just for colors.