Make MailSender non IO-blocking

[pablo]

Currently, scrapy.mail.MailSender is IO-blocking due to a bug in the (non IO-blocking) dispatching mechanism.

To quote the FIXME comment in scrapy/mail.py:

        # FIXME ----------------------------------------------------------
        # There seems to be a problem with sending emails using deferreds when
        # the last thing left to do is sending the mail, cause the engine stops
        # the reactor and the email don't get send. we need to fix this. until
        # then, we'll revert to use Python standard (IO-blocking) smtplib.

We need to fix this and make MailSender non IO-blocking and correct.

[pablo]

Done in r1955