Reinstated clean_pyc and added tests

I've reinstated clean_pyc, with a slight modification (did the same for compile_pyc). If verbosity level is > 0 and no path is given as a command line option, print out a message with the assumed project root. You can use the --path flag to set the directory to be cleaned. I've added tests to make sure everything works as intended.

The unreliable detection of the project root was to be the main reason why clean_pyc was removed (issue #503).