Added ability to specify **match-all** validation group, which
constraints will runs on every specified validation group.
Added groups="*" option to `Form::data` Valid validator.
In the dumped PHP class, we must use get() and not get*Service() methods to get services.
That's because all calls must be managed by get(). From the outside, you can call
get*Service() because as they are protected, they are caught by the __call() method;
which is not the case obviously when it is used internally.
If not, if you override a service with set(), this won't work when a service
depends on this one (the default one will still be used).
Some explanations on how it works now:
* The Session is an optional dependency of the Request. If you create the
Request yourself (which is mandatory now in the front controller) and if
you don't inject a Session yourself (which is recommended if you want the
session to be configured via dependency injection), the Symfony2 Kernel
will associate the Session configured in the Container with the Request
automatically.
* When duplicating a request, the session is shared between the parent and
the child (that's because duplicated requests are sub-requests of the main
one most of the time.) Notice that when you use ::create(), the behavior is
the same as for the constructor; no session is attached to the Request.
* Symfony2 tries hard to not create a session cookie when it is not needed
but a Session object is always available (the cookie is only created when
"something" is stored in the session.)
* Symfony2 only starts a session when:
* A session already exists in the request ($_COOKIE[session_name()] is
defined -- this is done by RequestListener);
* There is something written in the session object (the cookie will be sent
to the Client).
* Notice that reading from the session does not start the session anymore (as
we don't need to start a new session to get the default values, and because
if a session exists, it has already been started by RequestListener.)
