more accessor classes for the parser context, allow to switch on and check

* python/TODO python/generator.py python/libxml2-python-api.xml
  python/libxml2class.txt: more accessor classes for the parser
  context, allow to switch on and check validity
* python/tests/Makefile.am python/tests/error.py
  python/tests/invalid.xml python/tests/valid.xml
  python/tests/validate.py: attded more test and and added error.py
  which I forgot to commit in the last step
Daniel
diff --git a/python/TODO b/python/TODO
index 40fadf9..d1799fb 100644
--- a/python/TODO
+++ b/python/TODO
@@ -41,5 +41,7 @@
   found.
 - error redirections and preformat
 - handling of node.content
+- access to xmlParserCtxt and push mode
+   - needed for SAX too
 
 Daniel Veillard