Provide standalone ROOTCompare wrapping entire functionality including argument parsing

1. Both functionalities are contained in executable ROOTCompare
2. Installs ROOTCompare and setup to $INSTALLPATH/bin
3. $INSTALLPATH/bin/setup can be sourced to setup the environment to use from anywhere in the current terminal 3.1) Installation library path as well as this packages library path are added to (DY)LD_LIBRARY_PATH 3.2) $INSTALLPATH/bin is added to PATH
4. Command line arguments are processed more generically (see src/ROOTCompare.cxx) and behaviour could even be extended further.
5. Compiles at least with -std=c++11. However, if ROOT was compiled with later std this is used.
6. Change CMake version to 3.8.2 to support CXX_STANDARD 17