Add support for mapbox api v4

Mapbox API v4 requires an access token to fetch map tiles. 

This pull request adds support for this. When initializing your Mapbox layer, set access_token in the config options. 

Eg. 

var mapboxLayer = L.mapbox.tileLayer("mymapbox.map-fake123",{access_token:"mytoken-bringsallthemapstotheyard"});

Symptoms of this issue include basemaps not being displayed in the resulting PDF.