layerGroup -> map, dataLayer -> markerLayer

The layerGroup rename + removing auto seems like a good solution to the callback confusion. In situations where a callback is needed, the map 'load' event can be used instead.

I'm convinced about the dataLayer -> markerLayer rename. The key factor was realizing that the target of mapbox.js is the MapBox API, not a generic TileJSON implementation.

The examples cleaned up very nicely.

Ref #244.