Update default.rb

[CHANGE] better way of unsetting a default attribute;
[CHANGE] cleaner way of disabling and enabling auth;
[ADD] notice on readme about auth disabling; [CHANGE] rubocop compliance;
[ADD] rubocop settings; [CHANGE] readme formatting;
Merge pull request #14 from brtz/master

Added ssmtp.auth_enabled (default true) and a bit of cleanup