Add support for client SSL certs, SNI (if present in urllib3)

Now we are able to support out of the box authenticated client connections to etcd. We have a slight advantage here with respect to pyetcd, as it uses requests that establish ssl parameters at the connection level, instead than at the pool level. 

Also, if someone for some weird reason uses etcd behind a reverse proxy (I know it's foolish, but still) with SNI, we can support it.