Integrate training code with comet.ml

[EiffL]

It would be awesome to add an option to g2g-trainer to integrate training with a service like comet.ml. I'm tagging @ryanhausen on this, maybe you have thoughts on this. I'll try to give it a go when I have time.

[EiffL]

Apparently, we can just add custom comet.ml hooks to the TF Estimator API. Should be easy :-)

[ryanhausen]

Ya shouldn't be too bad :) It might be interesting to see if there is a clever way to use comet to keep a sort of leaderboard or something for different datasets using comet