mailutils: send_email() with attachments

* Extends mailutils.send_email() API to support attachments, following
  the recipe:
  <http://stackoverflow.com/questions/3362600/how-to-send-email-attachments-with-python>
  (closes #1253)
BibSched: new --email-logs-to bibtask CLI

* Introduces new --email-logs-to bibtask CLI, in order to automatically
  send stdout and sterr logs (and a status report) about a bibtask, at
  its very end.
  (closes #1252)
BatchUploader: bibtask logs via email

* Exploits the new --email-logs-to CLI interface of bibtasks, by letting
  the admins who schedule a batchupload (metadata or document) to specify
  a list of email addresses (by default her own) to whom the logs of
  the bibupload task are going to be sent.
  (closes #1255)