XML indentation with environment support

Complements #612 

This is a draft to get more suggestions. With this pull request I added a support to `indented` method to get a value from environment as discussed in #612.

Since this change affects `MockSerializationResult`, the constructor was deprecated and a new constructor with `Environment` parameter was added to keep compatibility backwards.

After this pull request, we can add the same support to JSON serialization.

Docs will improved before merge.
I think that I can't do this change, as Travis warning us. See https://travis-ci.org/caelum/vraptor4/builds/28048123 and https://travis-ci.org/caelum/vraptor4/builds/28048448.