Simplify TFClient and allow unsubscribe by key

Several simplifications + one features

You can now do

```js
  var tfClient = ros.TFClient({
    angularThres : 0.01,
    transThres : 0.01
  });

tfClient.subscribe('odom', function(tf) {
    ...
});

tfClient.unsubscribe('odom');
```

TODO implementing via event emitter?