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.
Hi @gwicke, I just found an error that I think it's caused by this line.
It seems that in te previous version, the parse function was called after removing the "?" from the this.search part, but now it's included, so the first querystring parameter has a "?" prepended.
Just changing this to `this.query = querystring.parse(this.search.substr(1));` should fix this, i think.