Add a prune() method to escape a tree from within a find() block

[thomasfl]

When used inside a 'find' block, a 'prune' method should skip the current file or directory, restart the loop with the next entry. Thus making the find method behave more like the find method in the find.rb standard library:
http://ruby-doc.org/stdlib/libdoc/find/rdoc/index.html