Issue #425 : Add custom layer

Adding a new "custom" type to allow user to create a custom Layer and use it in angular leaflet directive.
To use it : 

```
var customLayer = L.MyCustomLayer(withParams);
var leaflet_parameters = {
    center: {
        lat: ...
        lng: ...
        zoom: ...
    },
    layers: {
        baselayers: {
             layer1: {
                  name: 'myLayer',
                  type: 'custom',
                  layer: customLayer
             }
        }
     }, ...
};