initial commit of GeoJsonCanvasLayer

Situation
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Our geojson features are drawn on the map. In the case of Almere events, this means drawing 4000+ DOM elements and moving them around on zoom and drag. With the forthcoming Leaflet 0.8 the drawing is probably happening on the canvas, which brings the amount of DOM elements down to one, but 0.8 is not here yet and my rough estimate is that this is not going to happen in the coming weeks and might take up to 6 months (very rough). 

Problem
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Events do not perform. It is important to get the events performing and it might be valuable to get some experience with the canvas api.

Actions
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- [ ] Split data loading from drawing in LeafletVectorService.
- [ ] Enable the dataloader to download for multiple vector data sources (in one call or at least in one layer)
- [ ] Create a canvas layer like https://github.com/CartoDB/Leaflet.CanvasLayer or extend leaflets tiled canvas layer that draws stuff from the vectorService.
- [ ] Get click events working again.