Add fast path for simple URL parsing

This patch adds a fast path for parsing of simple path-only URLs, as commonly
found in HTTP requests received by a server.

Benchmark results [ms], before / after patch:
```
/foo/bar              0.008956   0.000418 (fast path used)
http://example.com/   0.011426   0.011437 (normal slow path, no change)
```

In a simple ab benchmark of a single-threaded web server, this patch
increases the request rate from around 6400 to 7400 req/s.
Sure @trevnorris,
```javascript
➜  node git:(master) node -v
v0.10.26
➜  node git:(master) node
> require('url').parse('/profile?access_token=1', true);
{ protocol: null,
  slashes: null,
  auth: null,
  host: null,
  port: null,
  hostname: null,
  hash: null,
  search: '?access_token=1',
  query: { access_token: '1' },
  pathname: '/profile',
  path: '/profile?access_token=1',
  href: '/profile?access_token=1' }
```
```javascript
➜  node git:(master) ./node -v
v0.13.0-pre
➜  node git:(master) ./node
> require('url').parse('/profile?access_token=1', true);
{ protocol: null,
  slashes: null,
  auth: null,
  host: null,
  port: null,
  hostname: null,
  hash: null,
  search: '?access_token=1',
  query: { '?access_token': '1' },
  pathname: '/profile',
  path: '/profile?access_token=1',
  href: '/profile?access_token=1' }
```
Look the `query` property in the parsed uri.