-Added Linux support (OSX support as well? More testing needed)
-Added material nodes. They are written to the .appleseed file as well.  A small bug: if two materials share a node tree, it will write duplicate nodes, causing errors when loading file.
-project_file_writer.pyx can be compiled for faster .appleseed file generation
Merge branch 'master' of https://github.com/appleseedhq/blenderseed