sets tigher bounds for map zooming

I've been getting a bit frustrated on my laptop as the map is a bit of a scroll vacuum stealing focus.  Mainly because it's got a fairly large zoom range.

You can still see the world and zoom pretty close.

Here are the new min and max zoom levels for reference:
![max](https://f.cloud.github.com/assets/711967/657561/843c97e4-d55e-11e2-9943-d03cc720adf6.png)
![min](https://f.cloud.github.com/assets/711967/657560/843018c0-d55e-11e2-9ce6-9e1c5dc2dc87.png)