Added support for putting the analysis results in directories inside the output file.

Added support for putting the analysis results in directories inside the output file.
The only change from the user side is in the format of the file name specified in:
AliAnalysisManager::CreateContainer().
Ex:
   mgr->CreateContainer("my_cont", TList::Class(), AliAnalysisManager::kOutputContainer,
                        "AnalysisOutput.root:mySpectra")
will write the output for this container in the folder mySpectra of the file AnalysisFile.root

Added a common file name as static data member to the analysis manager to be able
to communicate to user code a common choice for the output file name. Analysis wagons
will have to use this like:

  TString outputfile = AliAnalysisManager::GetCommonFileName();
  outputfile += ":myOutputFolder";
  mgr->CreateContainer(..., outputfile);