Linked favicon in index.html

168a9e3 added a favicon file, but it is not referenced in the html file. Firefox requires the link tag to find the favicon.

I already posted this pull request as #1443, but it got closed. The issues is not fixed for me, and [the codebase does not contain `favicon` anywhere](https://github.com/elasticsearch/kibana/search?utf8=%E2%9C%93&q=favicon), so I guess this is still open.