Basic initial implementation.

Here's an initial working version.  It was pretty simple to get the core tests from joyent/node working.  I also set up testling, but you'll have to configure it here and add the status image to the README.

[![build status](https://ci.testling.com/aearly/node-url.png)](https://ci.testling.com/aearly/node-url)

Predictably, it's failing in IE8 due to lack of ES5 support, but I'll leave that for later.