add requirements.txt file for pip
refactored the `default_path` function in `sheets`.

And added two functions to `utils`:

* `get_userdata_dir`

  returns the default user data directory

* `get_default_data_dir`

  returns the default directory used for user data

Now which directory is used to store the user sheets,
is determined the following way:

1. If the `DEFAULT_CHEAT_DIR` environment variable is set, use it.

2. If a `.cheat` directory exists in the home directory, use it.

3. Use a `cheat` directory in the systems default directory for user data.