[WIP] Change vector density when zooming

This change sets a number of vectors to show when at minimum zoom, and quadruples the number at each increased zoom level, as long as there are enough vectors available.  In practice, this doesn't span the entire zoom range, so at higher levels the density will decrease.

Performance is an issue at higher zooms.  Perhaps something can be done to avoid spending time updating vectors outside the frame?

Closes #7.
I think this will break anything that was manipulating the view or anything below it. The layer selection control for example is expecting a single layer (`velocityView.velocityGroup` in this case) to persist for the life of the map so that it hide it as necessary. Implementing some kind of safe `setOptions` method on `VelocityView` would insulate against this. The problem attributes are `.points`, which is an array of latlngs representing vector locations, and `velocityGroup` which is the parent `ILayer` containing all of the vectors.