update .event() for page and screen

[calvinfo]

thinking for those two, it just makes sense to include the fullName automatically:

Page.prototype.event = function(name){
  if (!arguments.length) name = this.fullName();
  return name
    ? 'Viewed ' + name + ' Page'
    : 'Loaded a Page';
};

since I think in most places we are doing that anyway

@yields, thoughts?

[calvinfo]

tippecanoe @ianstormtaylor too

[ianstormtaylor]

sgtm!