Applied patch from Brent Hendricks adding support for late DTD validation.

* python/generator.py python/libxml.c python/libxml2class.txt
  python/libxml_wrap.h python/types.c: Applied patch from Brent
  Hendricks adding support for late DTD validation.
* python/tests/Makefile.am python/tests/dtdvalid.py
  python/tests/test.dtd: integrated the provided regression test
Daniel

python/tests/dtdvalid.py

@@ -0,0 +1,32 @@
+#!/usr/bin/python -u
+import libxml2
+import sys
+
+# Memory debug specific
+libxml2.debugMemory(1)
+
+dtd="""<!ELEMENT foo EMPTY>"""
+instance="""<?xml version="1.0"?>
+<foo></foo>"""
+
+dtd = libxml2.parseDTD(None, 'test.dtd')
+ctxt = libxml2.newValidCtxt()
+doc = libxml2.parseDoc(instance)
+ret = doc.validateDtd(ctxt, dtd)
+if ret != 1:
+    print "error doing DTD validation"
+    sys.exit(1)
+
+doc.freeDoc()
+dtd.freeDtd()
+del dtd
+del ctxt
+
+# Memory debug specific
+libxml2.cleanupParser()
+if libxml2.debugMemory(1) == 0:
+    print "OK"
+else:
+    print "Memory leak %d bytes" % (libxml2.debugMemory(1))
+    libxml2.dumpMemory()
+