Added new command line option for selecting what call graph generation tool will be used and added logic to support that. Instance of loader (whether its a CflowLoader or GprofLoader) is now injected (i.e. passed as an argument to the constructor) into CallGraph and all call graph generation logic is delegated to the loader object. All occurences of 'source_dir' have been renamed to 'source'. This new name makes more sense now that the tool supports both files and directories as input.
Updated test suite. All tests pass.