mail support
add default sender capability, tune the APIs little bit
make the template rendering works
can render both the html & txt template, if exists
adjust the APIs a little bit
use Buffer as internal body, some api changes too
should encoding the mail subject correctly
add MailSender interface, with this, I can easily implment SendGrid mailer instead of smtp
add mock unit tests
typo...
Merge pull request #292 'arkxu/master' into develop