refactor(net): NetworkConnecter no longer is for static usage

Instead, you can use an instance of a NetworkConnector with
`Request::with_connector`. This allows overloading of the NetworkStream
constructors, so that it is easy to modify how an `HttpStream` is
created, while still relying on the rest of the stream implementation.

BREAKING CHANGE
Merge pull request #168 from hyperium/ssl-verify

r=reem