Added ConfigManager to preserve some values. Fixes #221

Well, mkstemp() is probably not what I want to use. I don't need to create any physical file. The '/dev/null' is just a dummy file name I pass to ConfigManager, the actual "physical" file is the fake file object created by flexmock. In load_configuration_file() there is:
        if not os.path.exists(self.config_file):
            return
I want this statement not to cause return, thus I am providing a file name which is guaranteed to exist. I believe '/dev/null' is existing in all UNIX platforms, that's why I chose this concrete name. Do you have better idea which does not need to create a physical file like mkstemp()?