Efficiency: Entity handling

Previously, used a regexp to find and convert named entities in the content.
Now use a more efficient algorithm.
Similar tweak for converting NCRs before checking whether text is valid utf-8.

test/unit/chunks/nowiki_test.rb

@@ -12,10 +12,4 @@ class NoWikiTest < Test::Unit::TestCase
 	)
   end
 
-  def test_sanitized_nowiki
-	match(NoWiki, 'This sentence contains <nowiki><span>a & b</span> <script>alert("XSS!");</script></nowiki>. Do not touch!',
-		:plain_text => '<span>a &amp; b</span> &lt;script&gt;alert("XSS!");&lt;/script&gt;'
-	)
-  end
-
 end