Add virtual resource tree

[Kaliumhexacyanoferrat]

As the developer of a web application, I would like to combine different sources of resources so that I am more flexible in serving content.

Example

var tree = ResourceTree.Virtual()
                       .Add("reports", ResourceTree.FromFTP(...))
                       .Add("charts", ResourceTree.FromWebDAV(...))
                       .Add("readme.md", Resource.FromWeb(...));

Host.Create()
    .Handler(Listing.From(tree))
    .Run();

Acceptance criteria

• The functionality is implemented in the IO module
• The builder allows any IResourceTree or IResource (or their builders) to be added
• Virtual trees can be chained (.add("section", ResourceTree.Virtual()))
• Acceptance tests are written