Added node throught package.json

Hello there,

I just added node functionality to the buildpack, it check if a package.json file exists and then runs this compile file ( I changed it a little ) : https://github.com/heroku/heroku-buildpack-nodejs/blob/diet/bin/compile

The heroku diet node compiler works nicely and caches everything it uses.

In this case, the node modules files are located in the $HOME folder ( `/app/node_modules/` ) and also cached.

Making the lessc command available at buildtime for example ( if you added less to your dependencies ).