Added CSS :nth() pseudo class

This is a Nokogiri extension (as far as I'm aware) but it's useful
enough to also include in Oga. Selectors such as "foo:nth(2)" are simply
compiled to XPath "descendant::foo[position() = 2]".

Fixes #123

lib/oga/css/parser.rll

@@ -473,6 +473,16 @@ even
     generate_nth_child('following-sibling', arg, current_element)
   end
 
+  ##
+  # Generates the AST for the `nth` pseudo class.
+  #
+  # @param [AST::Node] arg
+  # @return [AST::Node]
+  #
+  def on_pseudo_class_nth(arg)
+    s(:eq, s(:call, 'position'), arg)
+  end
+
   ##
   # Generates the AST for the `:first-child` selector.
   #

spec/oga/css/evaluator/pseudo_classes/nth_spec.rb

@@ -0,0 +1,21 @@
+require 'spec_helper'
+
+describe 'CSS selector evaluation' do
+  describe ':nth pseudo class' do
+    before do
+      @document = parse('<root><a /><a /></root>')
+
+      @root = @document.children[0]
+      @a1   = @root.children[0]
+      @a2   = @root.children[1]
+    end
+
+    it 'returns a node set containing the first <a> node' do
+      evaluate_css(@document, 'root a:nth(1)').should == node_set(@a1)
+    end
+
+    it 'returns a node set containing the second <a> node' do
+      evaluate_css(@document, 'root a:nth(2)').should == node_set(@a2)
+    end
+  end
+end

spec/oga/css/parser/pseudo_classes/nth_spec.rb

@@ -0,0 +1,15 @@
+require 'spec_helper'
+
+describe Oga::CSS::Parser do
+  describe ':nth pseudo class' do
+    it 'parses the :nth(1) pseudo class' do
+      parse_css(':nth(1)')
+        .should == parse_xpath('descendant::*[position() = 1]')
+    end
+
+    it 'parses the :nth(2) pseudo class' do
+      parse_css(':nth(2)')
+        .should == parse_xpath('descendant::*[position() = 2]')
+    end
+  end
+end