Added snapshot methods

[shnhrrsn]

• Adds support for Doorbot.snapshotTimestamps and Doorbot.snapshot
• Updates Doorbot.fetch to treat the response body as a Buffer rather than a string, which is necessary for snapshot images to be coded property

Usage:

ring.devices((e, devices) => {
  ring.snapshotTimestamps(devices.doorbots[0], (e, timestamps) => {
    ring.snapshot(devices.doorbots[0], (e, image) => {
      fs.writeFileSync('./snap.jpeg', image)
    })
  })
})

[schmupu]

Would be great to add the snapshot function into doorbot