Adding support for Download builder

Closes #789. As discussed in dev mailing list and under issue #789.

I don't know if we use all builder subclasses as inner classes or external classes. Ideas?

After all comments, I can write some docs and improve tests for bytearray and inputstream.
I don't like this kind of javadoc. This is hard to maintain and kinda heavy.
I think a succinct description would be enough here (and keep our docs
symmetry)... we can share this full explained code on VRaptor's site docs.