Add layers.json
Add a layers service

This service loads the layers metadata from the layers.json file, and provides a getLayerById function to get a an ol.layer.TileLayer object from a layer id.
Bg layer selector directive uses gaLayers service
Remove the gaWmtsLoader service
Remove WMTSCapabilities.xml
Copy layers.json to app-prod
Add tests for gaLayers service
Merge pull request #111 from geoadmin/dev_layers

Rely on MapServer service for layers