Dsa webworker

following pr #43

```javascript
if (Worker) {
    //create DSA key in background

    var worker = new Worker('/lib/dsa-ww.js')

    worker.onmessage = function(e) {
        var type = e.data.type;
        var data = e.data.data;

        if (type == 'debug') {
            console.log(data);
        } else if (type == 'data') {
            $(document).trigger('dsa.otr', DSA.parsePrivate(data.key));
        }
    };

    //start worker
    worker.postMessage({seed: BigInt.getSeed()});

} else {
    //fallback

    var dsa = new DSA();

    $(document).trigger('dsa.otr', dsa);
}
```