Add passing offsets over plotpan event

Unified this behavior with plotzoom event as in pull #34 (commit flot/flot@1b6c4e933ade3f03d634dfaa6357e82beda3bc55)

Again, synchronization of plots now can be done with:

```javascript
onePlot.bind('plotpan', function (event, plot, args) {
    anotherPlot.pan(args);
});
```