case sensitive matcher and default favicon location.

[fluxsaas]

maybe you want to add a case-sensitive matchers for looking up the favicon:

 ['link[@rel="Shortcut Icon"]', lambda { |el| el.attr('href') }],

https://github.com/fluxsaas/pismo/blob/master/lib/pismo/internal_attributes.rb#L36

also, it might be nice to add the default favicon location e.g:

example.com/favicon.ico

seems to be widely used...

thanks for the library!