Synchronization tool

Synchronizing multiple charts seems to be a common desire. This function in `extras` greatly facilitates it.

Usage:

```javascript
var g1 = new Dygraph(...),
    g2 = new Dygraph(...),
    g3 = new Dygraph(...);
var sync = Dygraph.synchronize(g1, g2, g3);
// the three charts are now synchronized
sync.detach();
// the three charts are now independent again
```

Demo [here](http://cdn.rawgit.com/danvk/dygraphs/8cadc6c95079212a5fd240616cc64cb474d25fb8/tests/synchronize.html).