adding tests for named_notebook_path

I want to do some refactoring, so I'll put in some tests that will define the
behavior that I want to preserve, so I can verify that my refactoring hasn't
broken anything.
cleaning up named_notebook_path
simplified named_notebook_path implementation

Also updated the tests
more named_notebook_path cleanup

named_notebook_path now returns path with leading and trailing slashes.
Work with @Zsailer
named_notebook_path: consistent usage convention
use new assumptions for named_notebook_path
Merge pull request #3 from ivanov/multidir

named_notebook_path improvements