Turned HTMLBody into a bytes.Buffer

Hi,

This switch allows you to use the message.HTMLBody as a Writer for html/template.

```go
var mailBodyTpl = template.Must(template.ParseFiles("NewsTpl.html"))

msg := gophermail.Message{
	To:      []string{t.Email},
	From:    "webmaster@somewhere.de",
	Subject: "News! News! News!",
}

// construct the args...

mailBodyTpl.Execute(&msg.HTMLBody, tplArgs)
checkErr(err)

// send the mail
```