[added] onClick handler to <Link />

See https://github.com/rackt/react-router/pull/173 for previous discussion.
This might be a little clearer as:

```
var ret = this.props.onClick(event);
// `false` prevents default; `undefined` does not
if (ret === false) {
  allowTransition = false;
}
```

and then just `if (allowTransition)` at the bottom; it wasn't clear to me on a first reading why you were comparing to false down there.