[xiaoqing] Continue Refactoring
[xiaoqing] Introduce apache commons long; Remove unused class UserEmail and SMTPAttachment; Introduce Address class.
[xiaoqing] Introduce slf4j; rename SendEmailUtils to EmailService.
[xiaoqing] Introduce class Recipients; Refactor by simplifying methods in EmailService.
[xiaoqing] Refactor by removing SMTPSession.
[xiaoqing] Remove unused properties in PropertiesEmail.