Add cx cy for bbox

In pull #674, I add new method for get midpoint of element.

without midpoint helper method, too much code need to write:
```
var bbox = ref.getBBox();
var path = "M" + pos.x + " " + pos.y + "L" + (bbox.x + (bbox.width) / 2) + " " + (bbox.y + bbox.height / 2);
```

better:

```
var point = ref.getBBox();
var path = "M" + pos.x + " " + pos.y + "L" + point.cx + " " + point.cy;
```

This pull remove the new method, but add `cx` and `cy` for bbox :)