Added XML::Node#before and XML::Node#after

These methods can be used to insert nodes before/after other nodes without
manually having to mess around with node sets.

This fixes #28.

lib/oga/xml/node.rb

@@ -142,6 +142,28 @@ module Oga
         return node_set.delete(self) if node_set
       end
 
+      ##
+      # Inserts the given node before the current node.
+      #
+      # @param [Oga::XML::Node] other
+      #
+      def before(other)
+        index = node_set.index(self)
+
+        node_set.insert(index, other)
+      end
+
+      ##
+      # Inserts the given node after the current node.
+      #
+      # @param [Oga::XML::Node] other
+      #
+      def after(other)
+        index = node_set.index(self) + 1
+
+        node_set.insert(index, other)
+      end
+
       ##
       # @return [Symbol]
       #

lib/oga/xml/node_set.rb

@@ -153,6 +153,20 @@ module Oga
         return node
       end
 
+      ##
+      # Inserts a node into the set at the given index.
+      #
+      # @param [Fixnum] index The index to insert the node at.
+      # @param [Oga::XML::Node] node
+      #
+      def insert(index, node)
+        return if @nodes.include?(node)
+
+        @nodes.insert(index, node)
+
+        take_ownership(node)
+      end
+
       ##
       # Returns the node for the given index.
       #