Enhance URL methods listdir(), files(), dirs(), walk(), walkfiles() and walkdirs().

listdir(), files(), dirs() have arguments include and exclude instead of
pattern (which worked like include does now). Also patterns can bow be lists
of strings.

walk(), walkfiles() and walkdirs() gained the same arguments. Additionally
the arguments enterdirs and skipdirs can be used to skip directories during
traversal.
Better description of the changes to URL methods. Add migration info.