Make dependencies optional

[tresf]

The goal is to move all legacy IE11 dependencies to optional dependencies, including RVSP

• Use native Promise's when available by default
• Use native Sha256 hashing when available by default
• Use fetch() over ajax (closes Replace $.ajax with fetch() #369) with IE11 pollyfill

[tresf]

@klabarge using this branch, can you take a look at https://github.com/qzind/tray/wiki/Signing-Messages and prepare the POST example to use fetch()?

[klabarge]

prepare the POST example to use fetch()?

Sure, I've got untested code here. I'll update the wiki once I've tested

[tresf]

Sure, I've got untested code here. I'll update the wiki once I've tested

Great, while you're at it, if you can test this branch (clean) that would be stellar too. It eliminates RSVP, bundles Sha256. qz-tray.js can now be a single JS lib without any dependencies (except maybe older frameworks missing WebSocket or Promise support)