Switch build system from Python/Java to pure JavaScript/Node/NPM.

As per the discussion here:

https://github.com/mrdoob/three.js/commit/9b8db498c8579763bf2dc7eebf4d40f9c891ac85#commitcomment-2471094

I have removed the Python-based build.py and the Java-based Closure compiler. I have also switched all build*.* scripts to call "node build.js" instead of "python build.py."

This unifies Three.js so that both its library and its build system are written in JavaScript.  It does not significant change the results as we ere not using the advanced features of the Google Closure Compiler.

I have modified build.js to give better error messages when "npm install" hasn't been run and I have added a README.md explaining how to get the build system up and running.

I am okay if this get's rejected, but I do think it is a simplification of the build system.