Hide single base layer in the Layers control

When only a single base layer is present on the map, i.e. listed in the Layers control, do not display it since there's nothing to choose from.

Customizable via the `hideSingleBase` option, defaults to `true`.

Example:

    L.control.layers({
        'Base Map': L.tileLayer(...).addTo(map)
    },{
        'Smile': L.tileLayer(...)
    },{
        collapsed: false,
        hideSingleBase: false
    });

![image](https://cloud.githubusercontent.com/assets/3540490/5488166/42cf8e2e-86ba-11e4-98c4-e00fbd9cea78.png)