Proper MIMEMultipart determination for smtp

For compatibility with most email clients, you cannot simply detail the message MIME as a MIMEMultipart-Alternative if you have embedded images within HTML. MIMEMultipart-Related is the correct format, per http://tools.ietf.org/html/rfc2387.

I have tested this with GMail, Thunderbird and Mail.app (Mac client), and it seems to work, whereas the current code will simply display my image attachment and none of the HMTL in Mail.app and Thunderbird.