applied patch from Alfred Mickautsch, bugfixes and comments added his test

* xmlwriter.c include/libxml/xmlwriter.h: applied patch from
  Alfred Mickautsch, bugfixes and comments
* doc/examples/*: added his test as the xmlWriter example
* doc/html/ doc/*.html: this resulted in some improvements
* include/libxml/hash.h: fixed an inclusion problem when
  <libxml/hash.h> wasn't preceeded by <xml/parser.h>
Daniel
diff --git a/win32/libxml2.def.src b/win32/libxml2.def.src
index c688c20..a6cd721 100644
--- a/win32/libxml2.def.src
+++ b/win32/libxml2.def.src
@@ -1134,8 +1134,11 @@
 xmlNewTextReader
 xmlNewTextReaderFilename
 xmlNewTextWriter
+xmlNewTextWriterDoc
 xmlNewTextWriterFilename
 xmlNewTextWriterMemory
+xmlNewTextWriterPushParser
+xmlNewTextWriterTree
 xmlNewValidCtxt
 xmlNextChar
 xmlNoNetExternalEntityLoader