Allow sites to specify favicon

[st0012]

• There needs to be an option (favicon_path?) to copy favicon to the output directory. static_path only copies files under the generator's folder.
• The option should be available in at least Rake task AND .rdoc_options. Not sure if practical to be a command line option, but I'm not against it.
• Templates should take this option into account and add a <link rel="icon" type="image/x-icon" href="/images/favicon.ico"> to the header when it's set.
• (Maybe) RDoc can provide a default favicon too?