export vertexgroups from blender

Hi @mrdoob,

I cleaned up my messy first pullrequest. Here is the old message:

I extended the blender-exporter to export vertexgroups when option vertexgroups is checked.
Did not find issues where this has been asked - but I needed it to distinguish which part of a mesh the mouse is over. Might have been solved with multiple meshes - I found this solution cleaner and perhaps someone else can use it ...

You can view the mesh I needed it for here: 3motic.com (with firefox it worked for me) .
I can provide it as example if you like.

There are only slight extensions with this feature - nothing else really changed - should not crash anything else ( I tested the newly build three.js with some of the provided examples ).