XML now have indented option

Closes #612.

Like Gson serialization, XML now is serialized without indentation by default. And an option to indented was added.

Note: after this pull request, I can add option to get from environment properties.
http://www.vraptor.org/javadoc/br/com/caelum/vraptor/environment/Environment.html#get-java.lang.String-java.lang.String-