Do not update signs on quit, and fix passing a nil value to format.
Do not pass a nil itemstack to on_rightclick
Merge pull request #27 from Pilcrow182/patch-1

Do not pass a nil itemstack to on_rightclick
add proper attribution and license info for the sounds used by the doors and
gates
add links to the sound files in question
local-ize a few variables
fix a copy&paste error, trim a couple redundant lines of code
gave doors simple wield images (same as inventory image)