Quick Browser Test Fix

The browser-based tests were not working right away for me, here are some changes I made to correct that:

 * Fixed the test-runner to use `superagent.js` instead of `build/build.js`
 * The "Content-Type params" test was broken (looks like an update to express change the behavior of charset in content-type headers recently)
 * Updated URL on README for `make test-server`