Support client/node encryption

This is a work in progress for #105. Most of the work is in Ione (see iconara/ione#6), the only thing that is needed in cql-rb is a way to get the `OpenSSL::SSL::SSLContext` into the connection options.

I've managed to run cql-rb against a node with client encryption enabled like this:

```ruby
cert_path = '/Users/theo/.ccm/encrypted_206/node1/conf/server.cer'
cert_store = OpenSSL::X509::Store.new
cert_store.set_default_paths
cert_store.add_cert(OpenSSL::X509::Certificate.new(File.read(cert_path)))
ssl_context = OpenSSL::SSL::SSLContext.new
ssl_context.cert_store = cert_store
ssl_context.verify_mode = OpenSSL::SSL::VERIFY_PEER

client = Cql::Client.connect(ssl: ssl_context)
client.execute("SELECT * FROM system.local")
```

You also need this in your Gemfile

```ruby
gem 'ione', github: 'iconara/ione', branch: 'ssl_support'
```

The API is not final, maybe the option will be called something different.