Expose native fetch in the browser

[jmrog]

This library has relied on fetch-ponyfill for quite some time now. fetch-ponyfill uses XMLHttpRequest under the hood. That is generally okay, but it is not okay for clients (like Studio) that want access to response headers on query execution requests immediately (i.e., before the query results have finished streaming back), in order to (for example) enable killing a query from the same editor where you initiated it. For that, we need to use native fetch (at least given how fetch-ponyfill works right now). What's more, we really probably ought to use the native fetch when possible anyway; stardog.js does not support Internet Explorer, and fetch has been released in all modern browsers for some time now.

Ideally, we would do this as part of a refactor of stardog.js -- see, e.g., PR #200 -- but in the interest of speed and making incremental steps, it is probably okay to do it in some incremental (even if temporarily hacky) way.

Internally, this is related to VET-634.

[jmrog]

Closed by #259.