Mouse handler api and point feature implementation

I'll have to add some tests when I get a change, but this seems to work.  The public api looks like this:

```javascript
points.mouseover(function (d, i, mouse) {
  // this: point feature instance
  // d: datum of the marker
  // i: index of the marker in the data array
  // mouse: mouse information object
});
```

There are handlers for `mouseover`, `mouseout`, `mousemove`, and `click`.  The nice part is most of the implementation is done generically in the `feature` class.  Subclasses generally just have to implement a
method to search for markers containing a given point.

Depends on #210.