Added a favicon - Githubissues

Merging, though we're probably not going to use it. My reasons are:

[ ] it's green. Gunn's colours are red
[ ] it's just a G on a circle; both don't really demonstrate much about Gunn other than "Gunn" starts with G
[ ] this favicon design isn't planned to be used around the site
[ ] the font for the G looks like Helvetica, and although it can be nice in some situations, this is not one of them
[ ] the G looks off-center
[ ] black on dark green?!

Orbiit / gunn-remake