Modify browserify task for use with multiple bundles.

@solomonhawk @nhunzaker

How's this look? (man the line comments are noisy... but helpful for people I think) Is there a more node-ier way to tell when to streams have emmited 'end' event? Might be overthinking it. This works fine... 

Also, do you think this is getting to complicated for a starter kit? Should I move this to an "Advanced examples" subfolder in tasks? ¯\\(°_o)/¯

Personally, I'd use this setup as my default.

Also, I'm going to refactor this a bit, and have a `shared.js` with jQuery and backbone in it, then have a `app.js` and an `admin.js` that rely on them, but exclude them from their bundles with the --external flag (see: https://github.com/substack/node-browserify#multiple-bundles).
Just merged the config.js abstraction. This can go in there now.