rewrite of onAdd method

Added feature so you can add a zoom level and center position to override the defaults of the onAdd method.

e.g.
```
    var m = {
        x: 1000,
        y: 1000
    };

    L.tileLayer.zoomify('/tiles-folder/', {
        width: 2560,
        height: 2560,
        maxZoom: 4,
        minZoom: 2,
        zoom: 4,
        center: map.unproject([m.x, m.y])
    }).addTo(map);
```