Added scent customization of paths to watch

Motivation/Summary
===============

I need sniffer to watch more than just current directory.
Therefore, made Sniffer recognize `watch_paths` from scent file.

Changes
=======

- Added property `ScentModule.watch_paths`

- Set attribute `ScentSniffer.watch_paths`. It looks for
  `ScentModule.watch_paths` and defaults to current directory.

- Used `ScentSniffer.watch_paths` in `main.run` instead of current
  directory.