[B] Escape Unicode characters when dumping YAML. Fixes BUKKIT-1466

# The issue:
Bukkit's YAML library, SnakeYaml, defaults to outputting unescaped Unicode characters. This causes Bukkit to produce invalid YAML files, which in turn cannot be loaded.

# Justification for this PR:
Implementing this change would ease plugin developers' work to support languages that use "special characters" and symbols.

# PR breakdown:
From SnakeYaml's documentation for setAllowUnicode():
> Specify whether to emit non-ASCII printable Unicode characters (to support ASCII terminals). The default value is true.
Setting this to false causes the Unicode characters to be escaped, and in turn Bukkit to produce a valid and loadable YAML file.

# Testing results and materials:
Loading this YAML file http://pastie.org/8380939 before applying this PR results in the following content once saveConfig() is invoked: http://pastie.org/8380940. Once the PR is applied, no apparent changes are made to the file upon saving.

# JIRA ticket
https://bukkit.atlassian.net/browse/BUKKIT-1466