Include more data in attachments

- Added ability to send extra data in attachments. This was modelled on
  the Mail notifier. The Job, Started, Finished and Duration fields are
  always sent in the attachment. If the `send_log` option is true, the log
  is also sent in the Detailed Backup Log field. The attachment color is
  one of the predefined Slack colors "good", "warning" and "danger".
- Added a helper method to test Excon invocations so that we can just
  match the payload fields that we are interested in.
- Updated all Hash operations to use the hash-rocket syntax
  instead of the newer 1.9 syntax.