[Mail] move isset/getter to bottom of Mail
[Mail] Change icebox order
Integrate swiftmailer
[Mail] add Usage section
[Mail] Refactoring + force contentType/charset
[Mail] mark 2nd US as done
Merge pull request #5 from ddd-php/swiftmailer-inte

[RFR][Mail] As a user, I should be able to send basic `TextMail` with SwiftMailer as `MailerInterface` implementation