Treemap layout now uses properties.

The inset property has been replaced by the standard top, right, bottom and left
properties; see the earlier commit that adds a convenience margin function to
mark. This commit also changes Treemap to use the standard pv.dom node
representation.

I also removed the magic to hide internal nodes if an inset (margin) is used;
it's probably better if users get in the habit of defining a visible function
to show only leaf nodes instead, e.g., visible(function(d) !d.firstChild).

TODO

@@ -7,16 +7,14 @@ data/Scale.js
 layout/Layout.js
 - allow interactive expanding / collapsing of tree nodes?
 - migrate all implementations to extend pv.Layout
+- use private data, or at least standardize which attributes are populated
 
 layout/Tree.js
 - allow depth and breadth offset to be computed automatically
 
 layout/{Cluster,Partition}.js
 - support customized inner/outer radius
 
-layout/Treemap.js
-- use pv.Dom.Node
-
 mark/Mark.js
 - antialias doesn't always work as expected; requires pixel rounding?
 - events property: "ignore", "all", "visible"