Pass custom dialer to http.Transport

[sergeyfrolov]

Recently we implemented long-requested access control lists feature #11, and added manual dialing in order to control that attacker could not get access to forbidden destinations with sneaky redirects in http responses.
Unfortunately, for all those nasty over-9000-http-requests workloads, we now establish separate tcp connection for each request. Establishing tcp connection to get favicon and then close is no bueno performance-wise.
We can solve this by passing a custom dialer(with built-in ACL) to http.Transport and letting HTTP package do socket pooling and reuse.