Set up scripts for creating symlinks and building

The main change here is to run a pre-install script to set up
symlinks, needed because npm doesn't install symlinks even if they're
present in the git repository.

Also kept the install script to "node-gyp rebuild". It sure doesn't
seem like this should be necessary (according to the documentation
node-gyp should run if the .gyp file is present), but it seems like
it's not getting built on Heroku without this explicit script set.
Remove node-glfw from dependencies
Change from GLFW backend to OSMesa backend (offscreen software rendering)

Based off of https://github.com/maghoff/osmesa-webgl

Note that the include/lib paths are hardcoded to /opt/mesa.

Note also that this doesn't play nicely with the GLEW/GLFW backend,
switching between them is not quite as simple as changing index.js.

It's possible we could do a better job of unifying here, but I'm not
sure. Some day GLFW might support OSMesa directly:
https://github.com/glfw/glfw/issues/281

Here's how I'm currently compiling Mesa (v 10.2.6):
./configure --enable-glx-tls --enable-texture-float=yes --enable-gallium-llvm --with-gallium-drivers=swrast --disable-dri3 --with-dri-drivers= --prefix=/opt/mesa --enable-gallium-osmesa --disable-egl
Update include/library paths for Heroku