Allow the items-node to have a list-value.

The items-node in the classes section now supports list values instead
of strictly string-values. This should make it easier to see exactly
which items a class has, and it should alleviate some of the problems
associated with apostrophes and such. The string-valued setup is still
supported to preserve some backwards compatibility.

The armor-node remains untouched, but it will probably be updated to
the same system later.
LinkedList -> ArrayList
Try to fix flight exception with Setup Mode.
Make showBlock(s) thread-safe.
Attempt to fix auto-start-timer issues.
Rip in peace, Metrics, you magnificent bastard.
Make spec warp mandatory.

This is necessary to ensure a robust /ma spec command. If no spectator
warp is set, the command will throw an NPE - it is much more safe and
sane to require the warp.
Revert "Rip in peace, Metrics, you magnificent bastard."

This reverts commit 096c4d4367113be5f19d0e62ec54a2c1c1657268.