BUG: Allow for the possibility that to_path is empty string.

This is a legal input for representing the root directory.
TEST: Expect a 404 on delete of non-existent file.
TEST: More fine-grained test for directory contents.

Verify that each entry in a directory listing matches the result of a
content-less direct get.
TEST: Add checks for subdirectory name splitting.

Previously the tests failed to catch a bug where a ContentsManager
didn't split name from path in a nested subdirectory.
Merge pull request #16 from quantopian/contents-test-reuse-2

Contents test reuse 2