Try to make autoscale=true a default

This ends up being more of a quagmire than expected - we initialize
tileLayers in L.mapbox.map and L.mapbox.tileLayer, while Leaflet expects
us to know retina status right then, and doesn't seem to support
changing this value after it is set. Would love some ideas from
@mourner about how this could become less hacky.
Merge pull request #781 from mapbox/autoscale-true

Try to make autoscale=true a default