Use the descendant axis for CSS selectors.

Instead of using "descendant-or-self" Oga will use "descendant". This ensures
that expressions such as "foo *" don't return a set also including the "foo"
element.

Nokogiri solves this problem in a somewhat different way by using //foo//* for
the CSS expression "foo *". While this works in Nokogiri the expression
"descendant-or-self::node()" is slow as a snail in Oga (due to its nature of
retrieving _all_ nodes first). By using "descendant" we can work around this
problem.

lib/oga/css/parser.y

@@ -45,7 +45,7 @@ rule
     ;
 
   descendant_or_self
-    : node_test { s(:axis, 'descendant-or-self', val[0]) }
+    : node_test { s(:axis, 'descendant', val[0]) }
     ;
 
   axis