add the .data() function and optimize invokation of both data() and selection()

[sttawm]

• Remove the tile.next.href logic (we weren't treating it correctly, anyway, as we were binding the fetched data, as opposed to all of the tile's data)
• Add the data() function
• Optimize data() and selection() such that they only are run on tiles with the results out-dated versions. Must invoke data() or selection() to update the version.
• API: The fetched features are stored on the tile as tile.fetched()
• API: A tile.draw function is now defined (delegating to geojson's private draw() function)

Testing:

• I've run a few .data() and .selection() functions, and panned / zoomed on the basemap layer.