TLS Support

This PR adds support for connecting to Docker servers using TLS. It adds the following flags to `docker-gen`:

* `-tlscacert file`
* `-tlscert file`
* `-tlskey file`
* `-tlsverify`

These flags correspond to the same flags on the `docker` client.

This PR also switches the handling of the event monitoring stream to use the support for event listeners built into `go-dockerclient`.

I did not yet add handling for the `DOCKER_TLS_VERIFY` or `DOCKER_CERT_PATH` environment variables, since I feel like the whole thing is a bit of a mess. In particular, it's not 100% clear which combination of environment variables should do what (e.g. how should you say that you want to ignore certificates on both ends but still connect with TLS? or when should a tool look for `~/.docker/cert.pem`, etc.).

It's pretty trivial to add this support before merging, but I want to move the discussion forward with some code.

Closes #31.
A similar problem exists if docker-gen is started before docker is running.  docker-gen says it's watching for events but it fails the first time and never retries.