v0.2

New features

• Add support for ESP event-tracking webhooks,
  including normalized AnymailTrackingEvent.
  docs
• Allow get_connection kwargs overrides of most settings for individual
  backend instances. (Can be useful for, e.g., working with multiple SendGrid subusers.)
  docs
• SendGrid: New SENDGRID_GENERATE_MESSAGE_ID setting
  controls workarounds for ensuring unique tracking ID
  on SendGrid messages/events (default enabled).
  docs

Other changes

• [possibly-breaking] Mailgun: eliminate automatic
  JSON encoding of complex metadata values like lists and dicts.
  (Was based on misreading of Mailgun docs; behavior now matches
  metadata handling for all other ESPs.)
• [possibly-breaking]: Mandrill: remove obsolete
  wehook views and signal inherited from Djrill.
  See Djrill migration notes
  if you were relying on that code.
• SendGrid: improve handling of 'filters' in esp_extra,
  making it easier to mix custom SendGrid app filter settings
  with Anymail normalized message options.
• Drop pre-Django 1.8 test code.
  (Wasn't being used, as Anymail requires Django 1.8+.)
• Mandrill: note limited support (because integration tests
  no longer available).