Handle backticks in _.escape/unescape

Fixes #1653

Matches [`Handlebars` escape map](https://github.com/wycats/handlebars.js/blob/88c52ded2e81b4b01d12c16e337b0bc4a453a813/lib/handlebars/utils.js#L4-11)
Something like this sufficient for you @michaelficarra?

```
test('_.escape', function() {
  equal(_.escape(null), '');
});

test('_.unescape', function() {
  var string = 'Curly & Moe';
  equal(_.unescape(null), '');
  equal(_.unescape(_.escape(string)), string);

// Don't care what they escape them to just that they're escaped and can be unescaped
test('_.escape & unescape', function() {
  var escapeCharacters = ['&', '<', '>', '"', '\'', '`'];

  _.each(escapeCharacters, function(escapeChar) {
    var str = 'a ' + escapeChar + ' string escaped';
    var escaped = _.escape(str);
    notEqual(str, escaped, escapeChar + ' is escaped');
    equal(str, _.unescape(escaped), escapeChar + ' can be unescaped');

    str = 'a ' + escapeChar + escapeChar + escapeChar + 'some more string' + escapeChar;
    escaped = _.escape(str);

    equal(escaped.indexOf(escapeChar), -1, 'can escape multiple occurances of ' + escapeChar);
    equal(_.unescape(escaped), str, 'multiple occurances of ' + escapeChar + ' can be unescaped');
  });
});
```