v0.6.0

CHANGES:

• Better implementation of STARTTLS (@dinosaure, #50)
• Properly quit if the server does not implement STARTTLS (@dinosaure, #51)
• Add let+ operator which manipulate result type (@dinosaure, #52)
• Upgrade fmt.0.8.9 (@dinosaure, #53)
• Be able to pre-allocate resources when we want to send an email (@dinosaure, #54)