Added id and top_texture parameters to createWood

This makes it easier when adding custom wood blocks and allows for
different top texture as well as side.

Tested with vanilla wood and some wood block form the Natura mod (http://www.minecraftforum.net/topic/1753754-).

I have a map rendered with my dev code, it includes this change plus some custom blocks.
Example of custom wood: http://www.donkers.net.au/map/#world/0/8/-151/155/64