Add method to allow application to check if URL is proxied

[tejaede]

Use Case: A snippet of javascript code is used to send requests to 2 URLs: http://proxied.org/query and http://origin-server.org/query

• http://proxied.org/query is polled and is sent with the x-retry-after header.
• http://origin-server.org/query is not polled and the endpoint does not accept the x-retry-after header.

Currently, the application can check if a URL will be proxied in the following way:

_isURLAccelerated: {
    value: function (url) {
        return !!XMLHttpRequest.configuration.getTransportUrl(url);
    }
}

A simpler and more understandable API would be to have a boolean such as one of the following examples:

XMLHttpRequest.isURLProxied(url)
XMLHttpRequest.configuration.isURLProxied(url)
XMLHttpRequest.isURLAccelerated(url)
XMLHttpRequest.configuration.isURLAccelerated(url)

[hthetiot]

See #40 (comment)