Add default favicon.ico

[brockovercash]

Just like there is a default 404.html and dirlist.html, I think it would be beneficial to serve a default favicon.ico for when users are using the site to serve simple files. If nothing else, it helps keep logs clean of 404s.

I think it could be easily implemented in server.go:HandleNotFound and I am happy to submit a PR with what I can come up with, but I wanted to hear thoughts first.

[cortesi]

Yes, please - this sounds like a good improvement, and I'd certainly merge a patch that implements it.