Implement canvas fingerprinting

[gabrielbauman]

It would be good to see an implementation of canvas fingerprinting (not the same as force-cached PNGS). See https://securehomes.esat.kuleuven.be/~gacar/persistent/index.html#canvas-results

[samyk]

https://github.com/kmowery/canvas-fingerprinting

Would be nice to have a "fingerprinting" portion of evercookie, canvas fingerprinting just being one method of it (obviously panopticlick methods are great and there are a number of other methods we can implement)