Re-added request service using request_stack one and returning current request

The WebProfilerBundle use the `app.request` key to be able to retreive the base url.

In Symfony the `app` twig global variable is an instance of `GlobalVariable` which contain a method `getRequest()`. This method internaly use the `request_stack` service.

But in Silex, the `app` twig global variable is the `Silex\Application` instance.
So we need to declare a service called `request` which internally use the `request_stack` service.

I don't know if another Silex service provider use the (old) `request` service.
I think It could be a great shortcut and it's one less migration to do for Silex 2.0.