Add MapBox Streets as a Layer

This issue is created so I can submit a patch to add the MapBox Streets layer. The patch is pretty simple and seems to be working fine in my limited amount of testing. (I thought there may be issues with API versions on the backend, but that doesn't seem to be the case).

This code was added to mapbox.layers.inc, function _mapbox_layers_info()

function _mapbox_layers_info() {
  $info = array();
  $info['mapbox_streets'] = array(
    'name'=> t('MapBox Streets'),
    'description' => t('MapBox Streets'),
    'layername' => 'mapbox-streets',
    'minzoom' => 0,
    'maxzoom' => 16
  );

Actual patch will be forthcoming