misc/email: improve, utf-8 aware headers, better logging.
misc/email: improve, utf-8 aware headers, better logging.

Some trivial improvements to email_send():
 - use pythonic logger
 - have one debug() line and one info() line before and after sending
   a message, respectively
 - if message is plain, don't do multipart, but simple email
 - if any header is non-latin, encode it with email.Header()
 - otoh, if mail is latin, don't encode it.

xrg/openerp-server,xrg/openerp-server