Update code to support express 4.

[schickm]

Currently a fresh install of audiostream breaks because it grabs the most recent express, which has been drastically reconfigured. Updating to run with express 4 is pretty easy and should only involve some changes in app.js. I'm happy to do the work and submit a pull request.

[nickdesaulniers]

Matt, yeah right on, I'd love a PR. I should give this lib some more attention, though it's somewhat obsolete now that FF is supporting mp3's now. Worst case, we could just set an upper limit on express versions in this repo's package.json file.