always forward attached files
use existing alot email_as_string function instead of email.message as_string method

The email.message documentation advises to use a generator and its
flatten method instead of directly using the as_string method of
email.message (see
http://docs.python.org/2/library/email.message.html#email.message.Message.as_string)

It's exactly what the alot crypto.email_as_string function does so we
use it instead.