Add loss-properties documentation.

[kumarkrishna]

Completes #74 and fixes #73 .

[Evizero]

very nice! I think we will have a merge conflict though because just two hours ago I made some changes to the convex documentation. I just pushed that. Could you maybe look into fixing the conflict on your end (with a rebase)? Basically what I changed is some typing and I added the reference to the Boyd book from which the definition seems to be taken out of

[Evizero]

actually I'll look into it right now

[Evizero]

Ok, thanks! I rebased your new commits and added them to the dev branch (so they are already merged now and I will close this PR).

[Evizero]

There seem to be some type-setting issues (which I may have introduced in the process). I'll take care of them. thanks again

[kumarkrishna]

Thanks! I've taken some definitions from the Steinwart book too, so it would probably be a good idea to put in a reference. Also, isclasscalibrated has a slightly more involved documentation, so I'll add that today and cover all the properties.

[Evizero]

I just pushed my changes to the properties to the dev branch. http://lossesjl.readthedocs.io/en/dev/losses/interface.html#properties-of-a-loss. mostly type-setting fixes, and some notation simplification