Mofidications in TBufferXML/SQL/JSON and TXMLFile classes

1. Equip TCanvas::Streamer to provide data members information for TBufferJSON/TBufferXML/TBufferSQL2 classes
2. Fix problem with streamer infos reading from TXMLFile
3. Fix error in TBufferXML/TBufferJSON/TBufferSQL2 when equipted streamers are used
   Reported also here: http://root.cern.ch/phpBB3/viewtopic.php?f=3&t=18802
   Error was introduced by modifications in I/O between 5-34/19 and 5-34/20
4. Implement TBufferJSON::CheckObject(), enable correct storage of colors palete in JSON with TCanvas
5. Provide missing documentation for some methods in TBufferXML/TBufferSQL2/TBufferJSON