[Translation] added method to expose collected message

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | #2435 
| License       | MIT
| Doc PR        | symfony/symfony-docs#3386
I see @fabpot suggested that. But I don't agree with that. First of it makes the argument type dynamic and secondly, there is no semantic difference.

So when you see code like

```
$translator->getMessages(array());
$translator->getMessages(null);
```

you would think both do something different and look at the implementation what the difference is. But in fact it's the same.