Introduced as_bytes for SafeMIMEText (and other SafeMIME-classes).

This is to provide a consistent interface (namely bytes) for the smtp
backend which after all sends bytes over the wire; encoding with as_string
yields different results since mails as unicode are not really specified.

as_string stays for backwardscompatibilty mostly and some debug outputs.
But keep in mind that the output doesn't match as_bytes!
Changed console and filebackend to use msg.as_bytes to output the data as it would get send via smtp.
Decode mails using the message encoding.
Merge pull request #2126 from apollo13/email_bytes

Many thanks to @bitdancer and @aaugustin for answering my stupid questions about (mail)encodings and pointing me in the right direction.