Nodesize configuration in test.
User message regarding variable importance.
Fix of division by 0 resulting into Float.NA
Fix of missing assignment leading to saving old CM.
A simple graph to show variable importance.

Spencer's code generating graph refined into two parts:
 - JS code providing a function drawing a SVG
 - CSS defining visuals of graph

The HTML code has to include JS as well as CSS (via <style
scoped>@import url('url')</style>

D3 library replaced by packed version to safe the network traffic ;-)
Removed nodesize since we have already min_rows param.
Var importance computed in parallel over all variables.
Tree stats.

Tracking min/max/mean for depth/leaves on the fly.