Layers added that load tiles sometimes do not invoke loading indicator

[ebrelsford]

See discussion in #15. Needs:

• Minimal example of failure. All we have right now is this one
• Tests to ensure that layers that do not load anything do not invoke the loading indicator, such as this one

[ebrelsford]

Minimal example of the loading indicator not working.

[jaslee]

My workaround for this to attach Leaflet.loading's own _show/_hideIndicator functions to each tileLayer's "loading"/"load" (i.e. started/finished loading tiles) event :-

myLayer.on('loading', function(e) { loadingControl._showIndicator() });
myLayer.on('load', function(e) { loadingControl._hideIndicator() });

[ebrelsford]

That's a nice enough solution, we should document this.