Add ability to cancel request

[NickGlenn]

Cancel requests and discard any promises/callbacks upon calling the cancellation method. Maybe called it abort() or cancel().

This may be supported by the Promise library being used.

[NickGlenn]

import { Portal } from 'portals';

const http = new Portal();

const req = http.get('/something-that-takes-forever');

req.cancel();

[NickGlenn]

This partially exists as of this commit: 8623962

Though it's not currently accounted for in the documentation or the test suite. The idea is that this is available on the promise returned by Portal.send().

const req = myPortal.get('/foo');

req.then( ... );

req.abort(); // invokes XMLHttpRequest.abort()

We can close this once we've extensively tested this functionality.