Include payload and extract fingerprint from payload

This will include an `extra` object in the assertion signing which is then extracted from the iframe in the `contents` variable.

The format of the extra object in the assertion is: 
`{ extra : { webrtc : { fingerprint: "<fingerprint_here>" }}}`

and can be found in the second `payload` (after the `~`) of the assertion:

assertion format: `header.payload.signature~header.payload.signature`