Update snakeyaml to 1.12; Adds BUKKIT-48, BUKKIT-3896

http://code.google.com/p/snakeyaml/wiki/changes
Issue 66 "Provide serialization with minimum layout change" is fixed in
snakeyaml 1.11 and addresses BUKKIT-48.

This commit also introduces an improved error message when a TAB is
attempted to be used for indentation.

Leaky: https://bukkit.atlassian.net/browse/BUKKIT-48
https://github.com/Bukkit/CraftBukkit/pull/1052