Hold gesture

This pull request adds Holds, BlueGel's first synthetic gesture! Hold tracks a hand held in place for a specified amount of time; this is useful for menu items (if you watch the position) and can be useful as a trigger mechanism (for instance, to prevent inadvertent gestures). The minimum duration and allowed movement (measured either in movement through space or in hand velocity) are configurable.

Holds can be listened to like any other gesture:

```js
var filter = new BlueGel.Filter();
filter.on("hold", options, callback);
```

I'll shortly integrate this in the LeapNoise demo app.