Python binding for xmlRegisterInputCallback

It is possible to make xmlIO handle any protocol by means of
xmlRegisterInputCallback(). However, that function is currently only
available in C API. So, the natural solution seems to be implementing Python
bindings for the xmlRegisterInputCallback.

* python/generator.py: skip xmlPopInputCallbacks
* python/libxml.c python/libxml.py python/libxml_wrap.h: implement the
  wrappers
* python/tests/input_callback.py python/tests/Makefile.am: also add a test case
diff --git a/python/generator.py b/python/generator.py
index 767c4bb..83d100c 100755
--- a/python/generator.py
+++ b/python/generator.py
@@ -339,6 +339,8 @@
         return 1
     if name == "xmlValidateAttributeDecl":
         return 1
+    if name == "xmlPopInputCallbacks":
+	return 1
 
     return 0