Allow abort network requests

Issue: http://code.google.com/p/phantomjs/issues/detail?id=230

This commit will introduce new argument in the `onResourceRequested` callback.
This new arg has type `JsNetworkRequest`. It has one public method - `abort`.
Using this arg user can abort the network request.

**Example:**
```
page.onResourceRequested(requestData, request) {
    // call request.abort(); to interrupt a network request
}
```