Short functions are pleasing.
Update docstring to contain usage information.

This change is done in preparation for a move from argparse to docopt.
For more information about docopt see:

    https://github.com/docopt/docopt

As of this commit the usage specifies that the commands 'copy', 'link'
and 'remove' are valid, they are not.
Prepare to use some modules, ignore '__pycache__'.
There should be one obvious way to do it.

Changes to usage instructions, they are still not valid.
Move from argparse to docopt.