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.
Merge pull request #572 from bfroehle/loader_usage_txt

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