Feature/yml 1

This PR is related to #1. 

This handler returns the specified favicon if r.ULR.Path == "/favicon.ico". I think that we should also probably add the following header : 

* `'Cache-Control': 'public, max-age=' + max-age`

The plan would be to change the signature from `favicon.Handler("favicon.ico")` to `favicon.Handler("favicon.ico", 86400000)`. Where 86400000 is a day in ms.

Is it looking like what you had in mind ?