XmlSerializationCodeGenerator enhancement

fix null issue in Write method; support different namespace in xml
attribute property like r:id
improve CT_Drawing serialization by using XmlDocument

get rid of XmlSerializer
fix notimplemented issue in CT_SheetPr
checkin latest .net 4 DLL
Merge branch 'master' of https://github.com/tonyqus/npoi
include test cases for XSSFConditionalFormatting
add two XSSF examples

chart is not working yet. This example is for experiemental purpose only
Merge branch 'master' of github.com:tonyqus/npoi