Implement favicon configuration and default

Thanks to @dansup for the default favicon, nodeatlas.png. The default
configuration points the favicon handler at it, but to install another,
one would simply put a file in res/icon (or symlink it) and specify its
filename with Map.Favicon.

Close #6
Close #7
Update .gitignore a bit

It now ignores user-installed favicons and .log files.