Default attribute assignment

I added the ability to specify default values for shader attributes. So, if the geometry doesn't have, say 'uv' data, and the shader uses it, the provided default value will be used for every vertex.
I added this only to ShaderMaterial right now. It could probably just be added to all Materials, I think.

I also added a member to ShaderMaterial called index0AttributeName. This allows the user to specify which attribute gets linked to attribute index 0 in the shader. The purpose of this is to avoid browser performance warnings when attribute 0 is disabled (due to the above ability to set defaults for missing data). By default, index0AttributeName is given the value of "position".