Add "mode" option support in case of download-only option set to true

We internally use a duplicate of this recipe with mode support. I would like to avoid using and maintaining duplicates internally, so I implemented it in this recipe.

Solve https://github.com/hexagonit/hexagonit.recipe.download/issues/14

Tell me if you are happy with the implementation / the tests.

As a bonus, add .gitignore file, as usual.
As a second bonus, add a new test egg requirement in Buildout > 2, "manuel". Easy to find with our beloved unittest module raising stupid exceptions when running h.r.download tests.