Add support for notify_from_email

[dotmanila]

Some email servers are somewhat restrictive about email senders specially on the same domain. Allowing to customize this value from SMTP can workaround this issue.

[06/07/2016 11:18:59] INFO: Mail has not been tested, sending initial test mail.
Traceback (most recent call last):
  File "./pyxbackup", line 3039, in <module>
    traceback.format_exc())
  File "./pyxbackup", line 1083, in _notify_by_email
    _die("Could not send mail ({0}): {1}".format(e.errno, e.strerror))
AttributeError: 'SMTPRecipientsRefused' object has no attribute 'errno'

recvfrom(4, "550-Verification failed for <roo"..., 8192, 0, NULL, NULL) = 261