Added configuration of attributions to layers of the map

Simply add an `ATTRIBUTIONS` property to the `LEAFLET_CONFIG` object to get you attribution on your layers.

    LEAFLET_CONFIG = {
        "TILES_URL": [
            ('main', 'http://{s}.tiles.mapbox.com/v3/examples.map-4l7djmvo/{z}/{x}/{y}.png'),
            ('detail', 'http://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png')
        ],
        "ATTRIBUTIONS": [
            ('main', 'copyrights mapbox'),
            ('detail', 'copyrights openstreetmap')
        ]
    }