Implemented RequestCallback feature that allows to abort Http Request

This feature improves jax-rs client api. It allows to create REST service via RestClient and pass RequestCallback as an argument. This RequestCallback is called when HTTP request has been initiated. And callback method receive Request as an argument. This allows to abort request.

For example:
````java
RestClient.create(MyRestService.class, new RemoteCallback<Entity>() {
    public void callback(Entity o) {
        // process response
    }
}, new RequestCallback() {
    public void callback(Request request) {
        this.request = request;
    }
}).call();
...
this.request.cancel();
````
Or more usable example with default implementation of RequestCallbackImpl
````java
RequestCallbackImpl requestCallback = new RequestCallbackImpl();
RestClient.create(MyRestService.class, new RemoteCallback<Entity>() {
    public void callback(Entity o) {
        // process response
    }
}, requestCallback).call();
...
requestCallback.getRequest().cancel();
````
This feature is very very necessary in my project. We have a lot of request that sould be canceled. For example live search queries.
Is this method needed / used anywhere?