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.