Manage all case we can do with the API using the backgroundLayers

The defaultBaseLayerRef isn't in backgroundLayers
The orthoRef isn't in backgroundLayers
We have only one layer who is a background layer
We have only one layer who is the ortho layer
JS lint
Merge pull request #614 from camptocamp/mapopacityslider-api

Manage all case we can do with the API using the backgroundLayers