Initial support new messages grouped by severity

As discussed in dev mailing list many months ago.

At this time we have only errors. These errors is a `List` of `Message`s. But some applications needs to messages like warning and info. We have a `Severity` enum with properties like `INFO` and `WARN`, but this messages is not used by VRaptor.

With this pull request we can simple add a `Message` into `Messages` managed bean. So in the view we can use someting like this:

```
<c:forEach items="${messages.errors}" ... />
<c:forEach items="${messages.warn}" ... />
```

Categorized messages are available too: `${messages.warn.from('client.name')}`, and more.

This change has backward compatibility because `messages.errors` is exported to view with alias `errors`, that is the same object. 

This is a initial suggestion, and we can improve it. After, javadocs and site docs will write.

Thank you.
Hmm, I'm not able to use `ListMultimap` because this class creates an `ArrayList`, but we need to create a `MessageList(ArrayList)` as you can see here: https://github.com/caelum/vraptor4/pull/736/files#diff-3c30e5f5d8b7bd61d2561087bc62a05fR41

Suggestions?