Use breadth-first-search for returning all nodes.

This still uses a stack but at least no longer relies on the call stack. I
decided not to go with the Morris in-order algorithm [1] as it modifies the tree
during a search. This would not work well if a document were to be accessed from
multiple threads at once (which should be possible for read-only operations).

I might change this method to actually perform a search (opposed to just
returning everything). This will require some closer inspection of the
available XPath axes to determine if this is needed.

Tests will also be added once I've taken care of the above.

[1]: http://en.wikipedia.org/wiki/Tree_traversal#Morris_in-order_traversal_using_threading

lib/oga/xml/document.rb

@@ -53,25 +53,25 @@ module Oga
       # Returns a NodeSet containing *all* the nodes in the current document.
       # Nodes are inserted in the order they appear in the document.
       #
+      # This method uses a breadth first search for tree traversal. See
+      # http://en.wikipedia.org/wiki/Breadth-first_search for more information.
+      #
+      # THINK: Turn into an actual search instead of returning everything?
+      #
       # @return [Oga::XML::NodeSet]
       #
       def all_nodes
-        return gather_child_nodes(self)
-      end
-
-      ##
-      # Recursively retrieves all child nodes of `node` and returns them as a
-      # node set.
-      #
-      # @param [Oga::XML::Document|Oga::XML::Node] node
-      # @return [Oga::XML::NodeSet]
-      #
-      def gather_child_nodes(node)
         nodes = NodeSet.new
+        visit = children.to_a.dup # copy it since we're using #pop below.
 
-        node.children.each do |child|
-          nodes << child
-          nodes += gather_child_nodes(child)
+        until visit.empty?
+          current = visit.pop
+
+          nodes << current
+
+          current.children.each do |child|
+            visit << child
+          end
         end
 
         return nodes