Add error handlers.

The error handler uses winston to log the error then passes it along,
while the default handler will respond using the correct MIME type
    (HTML, JSON, or plain text).
Filter out npm-debug.log.
Add initial tests.
Properly test that the responses are of the correct type.
Test the HTML output using cheerio.

Cheerio (cheeriojs/cheerio on GitHub) is a faux DOM model/jQuery
lookalike. This asserts that the displayed file items contain only the
expected items.