Having to call two methods to get complete feature information (id and data) is cumbersome. The `getFeatureInfo` method returns an object with both feature id and data.
The tile now has responsibility for resolving feature ids and fetching feature data given x, y pixel offsets with getFeatureId and getFeatureData methods. The layer has corresponding getFeatureId and getFeatureData methods that take a map location, lookup the appropriate tile, and delegate to the tile for the rest of the work.
I think it should be the job of the layer to retrieve data for a given location (instead of the control). The first part of this change creates a `getData` method on the layer and updates the control to use this method.
The second part of this change removes the assumption that the data returned will be an simple object representing feature attributes. The UTFGrid specification doesn't say anything about the structure of property values in the optional data member. The examples given in the spec use string values. The default callback previously assumed that the data could be rendered in a two column table. I think it would make more sense not to make this assumption. With this change, the user must always provide a callback to do anything with returned data.
