Add /drip, which drips data over a duration after an optional initial delay.

Useful to debug slow downloads.

Example

```
$ time curl -iN "http://lh:9090/drip?numbytes=10&duration=3&delay=1"
HTTP/1.0 200 OK
Content-Type: application/octet-stream
Access-Control-Allow-Origin: *
Connection: close
Server: Werkzeug/0.9.1 Python/2.7.1+
Date: Fri, 28 Feb 2014 09:59:41 GMT

**********
4.012 secs
```