Added very first draft of the Scheduler interface class

CMakeLists.txt

middletier/CMakeLists.txt

middletier/common/ArchiveToDirRequest.cpp

middletier/common/ArchiveToDirRequest.hpp

scheduler/CMakeLists.txt

scheduler/Scheduler.cpp

scheduler/Scheduler.hpp

tests/CMakeLists.txt

xroot_plugins/CMakeLists.txt