Use Pathnames and singleton for cookbook_paths

Make it possible to manipulate the list when using Librarian or Berkshelf.
Add a method to add cookbook paths
Add Librarian's install directory to cookbook_paths
Split cookbook path array construction to helpers
Add unit tests for config path helpers
Add unit test that Librarian path is added to cookbook_paths
Use single line block for constructing expanded config paths
Merge pull request #226 from tmatilai/auto-add-librarian-dir

Automatically add Librarian's install directory to cookbook_paths