Add ability to use CA Certificate to verify etcd server

At the moment this library won't allow mutual authentication when using the TLS transport, which somewhat defeats the purpose.

This change introduces the option `CaCertFile` so that the end user may provide an x509 PEM-encoded certificate to verify the server's certificate with.

This more or less catches the library up to the options `etcd` already exposes, allowing for full mutual authentication when using a client certificate over TLS.