Environment variables NODE_PREFIX, NODE_PATH in node-waf

Those variables have following defaults :
 - NODE_PREFIX is relative to the current path of the node_addon.py file
   It is used as the base path of node include files.
 - NODE_PATH is ~/.node_libraries
   It's where modules are installed when calling `node-waf install`
   Note .js files must be explicitely installed by the module's wscript.

Usage :
NODE_PREFIX=/usr/local NODE_PATH=~/.node_libraries node-waf configure