hover twitter share button

it is a bit different to all others. – so maybe it would be better to have:

#top #wrap_all .av-social-link-twitter a:focus, 
#top #wrap_all .av-social-link-twitter:hover a {
  color: #fff;
  background-color: #000;
}

This seems to be in line with twitter’s corporate design – and is also similar to the behaviour of all other share links on enfold.