Use NODE_MODULE to create node_gd binding

Had to rename from node-gd (to node_gd) so the NODE_MODULE macro can correctly
create the module struct. Updated the require()'s to reflect this.

Signed-off-by: Tim Smart <tim@fostle.com>
Merge pull request #5 from tim-smart/bug/node-v0.10

Use NODE_MODULE to create node_gd binding