Clamp north south

@jbeezley  please review this branch. Apart from fixing the zoom it fixes some othe major issues as well. Now it respects the proper placement of the map given a zoom level and the center. So now zoom level 3 should look identical to zoom level in in openlayers (images below: left geojs, right openlayers). 
![geojs](https://cloud.githubusercontent.com/assets/146527/3345158/590435d0-f8b4-11e3-87e1-f6059efedff3.png)
![ol](https://cloud.githubusercontent.com/assets/146527/3345159/5c847b66-f8b4-11e3-9767-a77fefa31f72.png)


Should we
```javascript
else {
  return null;
}
```
after this?