Add support for the 'members' command, other small fixes

This adds support for the 'members' command, including a simple test, and a couple of other small improvements.

Deserialising serf RPC responses is a little cleaner now without the BytesIO object, and the SerfClient class is available when the module is imported, avoiding the need to import serfclient.client.

Sorry, these should probably be in separate pull requests but it didn't seem worth the effort! :)