Rewrite the sandbox implementation

[czocher]

The sandbox now requires some path tricks for it to work. It'd better if the language definition in the settings.py file would be generic (not sandbox-specific). First thought is to change the Judge run command/the Sandbox execute command and a param like "run_path" or something. Then perform a os.path.join(sandbox_path, run_path)+runCommand.
Maybe language-specific modules should be created?