Added a readNodeURL callback
Modified PagedLOD to call to readNodeURL, just in case there exists a custom readNode callback
Adds better shader error reporting (surrounding lines + error listed in console)
Merge pull request #170 from cedricpinson/feature/shaderFailBetterLog

Adds better shader error reporting
Adds example to show how to debug scenegraph

This example need to be added as option in osgViewer to use it
dynamically on arbitrary scene.
Also a lot of improvement are needed:
 - add small button a la 'glDebugger' instead of big one
 - add more informations about nodes
 - improvement rendering of matrixes
 - handle better how d3/dagre can be loaded only when needed
Merge pull request #169 from jtorresfabra/readNodeCallback

Added a readNodeURL callback
Fixes codepath to load assets relative to node url