Add StalkerConfiguration class

- Move methods into DefaultStalkerConfiguration

- Use concrete types for generics for type safety

- Fix cases where String was cast into File instead of using an
  instance of File in test and source

- Use absolute path comparisons instead of path to cover cases
  where .path is equal to an absolute path.

- Add tests for both StalkerConfiguration and
  DefaultStalkerConfiguration.

- Fix bug where leading slash was added to printDirs output