Subtract 100 pixels from target.offset().top. like so:
$(document).ready(function() {
$('a[href^="#"]').click(function() {
var target = $(this.hash);
if (target.length == 0) target = $('a[name="' + this.hash.substr(1) + '"]');
if (target.length == 0) target = $('html');
$('html, body').animate({ scrollTop: target.offset().top-100 }, 1000);
return false;
});
});