Interfaced MailGun to allow to be easily replaced

[olivmaurel]

• Controllers can use the SmtpAwareTrait to inherit the smtpSendMessage() function
• Wrapped MailGunMailer into a simple class SmtpMailerMailGun interfaced to SmtpMailer
• Bumped MailGun version to 3.0

[esokullu]

Thank you Olivier! But in this current version, Mailgun is not really SMTP it is using REST APIs, so it should be renamed as such.

You did well by creating a trait and abstracting it away from the rest of the stack, but as of now, it is too abstract it can be used for SMTP or anything (SendGrid APIs, Mailgun APIs)

I think this structure is good, as long as the names reflect this attribute properly. We'd also need a second adapter with plain PHP SMTP (without relying on 3rd party requirements.)