Use `nose.__loader__` to load 'usage.txt', if available.

Previously we only used `nose.__loader__` if we thought that the loader was zipimporter. Additionally, we use `nose.__file__` to build the path to 'usage.txt' as suggested by [PEP-302](http://www.python.org/dev/peps/pep-0302/#optional-extensions-to-the-importer-protocol).