Remove jQuery dependency

[mejackreed]

• xhr (see replace getJSON with pure XHR/JSON.parse implementation #45)
• promises

[epoz]

I made another attempt at: epoz@88fa7ed using the fetch() API.

@mejackreed comments on Slack:
"Without any arrow function support for Internet Explorer, I think this may be a non-starter until that can come. I would be more inclined to adopting something like the approach in #45 for maximum compatibility."

My goal with this is to use leaflet-iiif in a React app, where I do not want to also load in jQuery just for this one component.

[abrin]

would you be more open to pulling out jQuery now that two years have passed? I wonder if the active browser support has rolled along enough that the older browser versions aren't as much of an issue (IE 11 at ~2% according to https://caniuse.com/#feat=arrow-functions)

[mejackreed]

@abrin yes. See #77 for the implementation. I was hoping to get some feedback on that.

[mejackreed]

Resolved in #77