Not in the "async/fibers/coro" sense of flow control, but in the TCP
backpressure sense.

Pause the stream when a write isn't flushed, and then resume it once the
writable stream drains.

Detach listeners and clean up buffer on error

[api] Expose adapted version of `stack` so it can be used with HttpProxy instances not created by `httpProxy.createServer()`

closes #80
Conflicts:
	lib/node-http-proxy.js

The number of maxSockets has to be set after the agent is created. Setting the property in the constructor does not work.