augmented types supported a number of new bug fixes and documentation

* gentest.py testapi.c: augmented types supported
* HTMLtree.c tree.c xmlreader.c xmlwriter.c: a number of new
  bug fixes and documentation updates.
Daniel
diff --git a/tree.c b/tree.c
index 81ad0ce..134ce20 100644
--- a/tree.c
+++ b/tree.c
@@ -242,6 +242,7 @@
     int len = 0;
     xmlChar *ret = NULL;
 
+    if (prefix == NULL) return(NULL);
     *prefix = NULL;
     if (name == NULL) return(NULL);