Implement attachments.

[qbit]

• Allow 'file' to be passed into Pushover.send(). This can either
  be a filename (string) or an object ({name: 'asdf.png', data: Buffer}).
  If you pass in the object, you can overwrite the 'content-type' header
  by setting something like 'type: "image/bmp"'.

This closes #23 and #20