Implements WHATWG URL support. Example:
```
var u = new url.URL('http://example.org');
```
Currently passing all WHATWG url parsing tests and all but two of the
setter tests. The two setter tests are intentionally skipped for now
but will be revisited.
PR-URL: https://github.com/nodejs/node/pull/7448
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
