[Validator] Added 'Any' validator

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

The `Any` constraint allows you to re-use existing constraints and is satisfied when at least one of the constraints validated.

### Usecase
You have a form with a field that can either be a number greater than `3`, or match a specific regex.
You could write your own `Validator` class and create your own little `$number > 3`-tests and `preg_match` calls. Or... you might be using an existing Constraint (e.g. `Iban`), and would simply like to also allow a different value to be passed (for instance a normal bank account number).

So how can we combine those constraints to satisfy a given value if **any** of them validates?

Here's where the `AnyValidator` comes in handy. You can pass it an array of constraints that will then be checked during validation. If **any** of them found the value to be valid, the constraint is satisfied (no errors). 

Below is an example of what this would look like in your FormBuilder:
```php
// src/Acme/DemoBundle/Form/Type/MyForm.php
public function buildForm(FormBuilderInterface $builder, array $options)
{
    // ...
    $builder->add(
        'my_example', 
        null,
        array(
            'label'       => 'My example field',
            'constraints' => array(
                new Any(array(
                    new Regex('/here-is-my-regex/'),
                    new GreaterThan(3),
                ),
            ),
        )
    );
    // ...
}
```

The validation message that is used when all constraints failed can be controlled by passing it as a key/value in the `$options` argument. For example: 
```php
// ...
$constraint = new Any(array(
    'constraints' => array(
        new Iban(), 
        new DutchBankAccount()
    ), 
    'message' => 'You must supply either an iban number OR a valid Dutch bank account'
);
// ...
```

Once this PR has reached an acceptable state I will add the necessary docs to complement the changes.

it should not be marked as part of the stable API (we haven't even yet accepted it as part of the API)