Pointer plugin

Allows panning a map via mouse dragging, and zooming via scrollwheel.

**pointerConfig**

```
 pan [Boolean] : Enables mouse panning.
 panBounds [Boolean] : Don't allow pan beyond visible elements
 zoom [Boolean] : Enables scrollwheel zooming
 zoomIncrement [Float] : % to zoom in each wheel unit
```

I couldn't test against IE, but when I first wrote most of this code things worked ok - I'm going to have to lean on you to re-verify ;)

Feel free to make any changes you want.