Fix from Philippe.

If the operator ">>" is required and not provided by the user,
then he get an error message.
If I/O is required and the '!' is used  and operator>> or operator<< is
missing, then an error message is printed.
The forward declaration of these operators has been moved back
to Rtypes.h. It was in TBuffer.h but TString.h does not include
TBuffer.h


git-svn-id: http://root.cern.ch/svn/root/trunk@4727 27541ba8-7e3a-0410-8455-c3a389f83636