each function that needs slk should check whether slk is available; same for slk_helpers

Use these lines:

import shutil
from pyslk.exceptions import PySlkException

if shutil.which('slk') is None:
    raise PySlkException("pyslk: slk: command not found")

if shutil.which('slk_helpers') is None:
    raise PySlkException("pyslk: slk_helpers: command not found")