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.
`errors` is generic too, and we use since 3.0 version. I vote to leave as `messages`. Or if more people vote to rename, instead of `messages` we can export `warnings` and `info`, or a better name than `vraptorMessages`, that is too ugly and we need to use upper/lower case, because is too boring to type in JSP.