[api breaking] Begin refactor to optimize node-http-proxy by managing one instance of HttpProxy per `host:port` location

[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.

This adds a flag to ProxyRequest to disable the setting of x-forwarded-[for|port|proto]
(will change default to true in next commit)

closes issue #72