Add Python classifiers for all supported versions

Thanks to @ThomasWaldmann for the original patch. His fork is gone so I
can't apply the original commits.
diff --git a/setup.py b/setup.py
index 2cf3106..3bfa749 100644
--- a/setup.py
+++ b/setup.py
@@ -66,7 +66,11 @@
         'License :: OSI Approved :: BSD License',
         'Operating System :: OS Independent',
         'Programming Language :: Python',
+        'Programming Language :: Python :: 2',
+        'Programming Language :: Python :: 2.6',
+        'Programming Language :: Python :: 2.7',
         'Programming Language :: Python :: 3',
+        'Programming Language :: Python :: 3.3',
         'Topic :: Internet :: WWW/HTTP :: Dynamic Content',
         'Topic :: Software Development :: Libraries :: Python Modules',
         'Topic :: Text Processing :: Markup :: HTML'