Configs deeply frozen to prevent accidental/malicious alteration.
Sanitize::Config::DEFAULT moved to sanitize/config/default.rb

This is consistent with basic, relaxed and restricted configurations.
Sanitize::Config.freeze_config
Merge branch 'freeze_config' of git://github.com/pda/sanitize
Unindent history.
Loosen Nokogiri dependency back to >= 1.4.4. Fixes #79
Release 2.0.5.