Pushover image attachment feature

[damoclark]

• Bugfix (non-breaking change which fixes an issue)
  and
• New feature (non-breaking change which adds functionality)

Proposed changes

This PR fixes a trivial bug and implements the new image attachment feature of the pushover API.

Bugfix

There is a small bug in the event handler passed to the input event on line 32. Inside the event handler function, it makes references to this intended for the outer scope (not the new functional scope within the handler).

New Feature

The image attachment feature looks for an attachment property in the NR message payload. This property can hold a local file path, or a Buffer (binary or base64 encoded) representing an image to be sent along with the message to the Pushover API server.

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the mailing list/slack team.
• I have run grunt to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality

There are currently no existing unit tests for this node. I have discussed this with @dceejay who has agreed to review the PR without unit tests.

[coveralls]

Coverage remained the same at 66.256% when pulling 5c5db9a on damoclark:pushover_attachment_feature into 73e4ae5 on node-red:master.