Revised attribution

Hits https://github.com/mapbox/mapbox.js/issues/744

- Attribution control is now default over of infoControl.
- infoControl is now triggered when it's set to true in the `L.mapbox.map` options param or when passed as an argument to `map.addControl()`.
- __Improve this map__ link now tracks on the attribution control

cc; @tmcw for a review.