Added ability to filter incoming requests

[nicDamours]

Summary

Added the ability to filter incoming requests, in case some of them doesn't have to be capture in Amazon Xray. This can be useful to reduce noise when using a health check url.

To add a new filter, simply call the addRequestFilterCallback function, on the Xray facade, and pass a callback. The current request will be evaluated against this callback. If the callback returns false, the XrayServiceProvider will return before registering the collectors.

I've added informations about this new feature in the doc. If you could, kindly, review the doc to make sure I havn't done any grammatical errors, I would highly appreciate it. English is not my primary language.

I've also added tests for the newly added behaviour.

Closes #44

Steps to tests.

1. Load a project with the library and connect it to Amazon XRay.
2. In your AppServiceProvider, add a new filter, in the register function, like so:

use Napp\Xray\Facades\Xray;
use Symfony\Component\HttpFoundation\Request;

class AppServiceProvider extends ServiceProvider
{
    public function register()
    {
        Xray::addRequestFilterCallback(function (Request $request) {
            return $request->getRequestUri() !== '/some/url';
        });
    } 
}

3. call the url you don't want to capture, a couple time, so it would have been sent to Amazon Xray
4. call another url that is not filtered out.
5. Only the request that is not filtered out should be capture in Amazon Xray.