Use DefaultServlet instead of ResourceHandler to serve static files.

[notthetup]

perwendel/spark#316

https://stackoverflow.com/questions/20207477/serving-static-files-from-alternate-path-in-embedded-jetty/20223103#20223103

https://stackoverflow.com/questions/34100735/serve-static-content-on-a-per-servlet-basis

https://stackoverflow.com/questions/39011587/jetty-default-servlet-context-path

[notthetup]

While we're doing this, we should also add a flag to enable directory listing of a context.

[notthetup]

While we're doing this, we should also add a flag to enable directory listing of a context.

This bit was fixed in #329