jake/building elFinder from source: support modern UglifyJS

- updated the Jakefile.js jake build script and environment to support modern uglify-js (the API of that one has been changed; see also https://github.com/mishoo/UglifyJS2#the-simple-way and http://stackoverflow.com/questions/13919782/uglify-js-api-throws-typeerror-when-executing-example-from-github )

- also makes sure the ./build/ directory exists already from the very beginning: no surprises. (This is accomplished by having a small .gitignore file in there that serves a useful purpose too!)

- Added package.json for `npm install` so you can load uglify-js and csso locally as they should.

Hence procedure to build from source:
          git clone
          sudo apt-get install node-jake   -- if you're on Debian Linux
          npm install
          jake -C ./build elfinder

tested on a Ubuntu 13/64 VM. `jake clean` also works as expected.