added a more modern favicon

I'm not the author of this favicon,
it comes from the link on opam.ocamlpro.com to ocaml.org
http://opam.ocamlpro.com/ext/img/ocaml.png
(I've edited it a little in Gimp though to optimise it a little
for a very small favicon size.)
typo

(PS: the .ico format instead of .png provides compatibility with MS/IE)
Merge pull request #170 from blue-prawn/master

Replace the old fashioned favicon from the old website with a more modern one.