add support for building polymer.concat.js, using code from platform-dev

I've been building platform.js using `grunt minify audit`, which helpfully produces a platform.concat.js I can use for debugging. My hope was to get something similar for polymer.js, so I took some code from https://github.com/Polymer/platform-dev/blob/master/gruntfile.js and adapted it.

@azakus -- mind taking a look?