This comparator can be used by collections that wish to support locale-based
sorting.  The locale specifies how upper and lower case are compared.
function localeComparator(model1, model2) {
    var a1 = model1.get(this.sortField),
        a2 = model2.get(this.sortField);

    if (a1 !== undefined && a1.localeCompare) {
        var result = a1.localeCompare(a2) * this.sortDir;
        if (result || !this.secondarySortField) {
            return result;
        }
        a1 = model1.get(this.secondarySortField);
        a2 = model2.get(this.secondarySortField);
        return a1.localeCompare(a2) * this.sortDir;
    }

    return a1 > a2 ? this.sortDir : (a1 < a2 ? -this.sortDir : 0);
}