Allow user to set min/max zoom levels and restrict extent for base layer

Added support for setting min/max zoom levels for maps using spatial layers as the base. That is, Google/Stamen/OSM baselayers are supported, but I didn't implement for WMS (I can work on that if someone has a working WMS server I could use for testing. All the WMS servers I know only deal in UTM coordinates).

I used the Styles tab to house the new options. It is probably too cluttered now, but I was thinking maybe starting with these new options being "hidden" initially and then viewable by clicking "more options" or something along those lines would maintain the clarity that was there previously. I thought I'd get some feedback before pursuing that.

Hopefully my locating of the logic in the _initSpatialLayers function isn't too weird, but I can refactor!

Let me know how else I can help with the PR process (I'm a bit of a noob, if you can't tell). 

![screen shot 2014-06-04 at 4 33 53 pm](https://cloud.githubusercontent.com/assets/666141/3180334/f8d6b0ce-ec2f-11e3-9ec1-2a669536d8a0.png)