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 an initial suggestion, and we can improve it. After, javadocs and site docs will write.

Thank you.
Then we should also rename warnings and info, too. 

In https://github.com/caelum/mamute, for example, we had were including an attribute named "messages" in our views and we broke our app when updated vraptor :-(

I think that this will happen for other users, too.