Generates a UUID.
Based on: http://stackoverflow.com/questions/105034/create-guid-uuid-in-javascript
function uuid() {
        var base = 'xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx';
        var result = base.replace(/[xy]/g, function (char) {
            var random = cryptoRandom16();
            var result = char == 'x' ? random : (random & 0x3 | 0x8);
            return result.toString(16);
        });
        return result;
    }