Include html5lib for BeautifulSoup

BeautifulSoup needs lxml or html5, have included html5lib.

Also latest BeautifulSoup 4.1.3

html5lib/serializer/xhtmlserializer.py

@@ -0,0 +1,9 @@
+from htmlserializer import HTMLSerializer
+
+class XHTMLSerializer(HTMLSerializer):
+    quote_attr_values = True
+    minimize_boolean_attributes = False
+    use_trailing_solidus = True
+    escape_lt_in_attrs = True
+    omit_optional_tags = False
+    escape_rcdata = True