Riak 2 Feature: Security

The Ruby client needs to support security per basho/riak#355 .

- [x] configure local and tls credentials

```ruby
c = Riak::Client.new(
                     pb_port: 17017,
                     authentication: {
                       user: 'user',
                       password: 'password',
                       server_ca: File.read('ca.crt'),
                       server_cert: File.read('server.crt')
                     }
                    )
```

- [x] upgrade a protobuffs connection to tls
- [x] kill the connection if the server doesn't have a valid cert
- [x] kill the connection if the server doesn't support tls
- [x] kill the conncetion if the server's cert is revoked
- [x] work correctly when requests succeed

(copied from #110 )