Improving oldIE Support

oldIE (in our case, IE8) is a pain to support sometimes

 * Chai.js has been dropped in favor of expect.js (since Chai uses ES5 features unavailable in oldIE)
 * ISO-8601 date parsing (via polyfill) for when `Date.parse()` fails
 * Adding a new server (`make server`) for development/testing (it's a simple [express](http://expressjs.com/) server w/ [simple-http-proxy](https://github.com/flokk/simple-http-proxy))
    * no longer requires an external dependency for development
    * includes an optional `/api` proxy so oldIE can run tests even w/o CORS
 * Created a fork of superagent that allows for IE8 to send `PATCH` requests (see visionmedia/superagent#262)
 * `DEVELOPERS.md` has been updated with this new information