Fix TLS events

Event monitoring is not working under TLS because it isn't using Client.HTTPClient. This patch fixes it.

I would have added a test, but `httptest.NewTLSServer` doesn't support TLS mutual authentication and my skills are not up to writing a full replacement.
That seems like a reasonable suggestion. I went with the approach I did based on [this comment](https://github.com/fsouza/go-dockerclient/issues/173#issuecomment-60669906) by @fsouza.

I'm happy to update this if there's some agreement to take your approach (or some compromise or other approach), but I mainly want the `DOCKER_HOST=tcp://X.X.X.X:2376` form used by the `docker` client and `boot2docker` to be supported without alteration as the endpoint argument to `NewTLSClient`.