Add version and active flags to series

Once this has been merged I'll rewrite the serialization tests in a way similar to https://github.com/monetas/btcwallet/pull/50
Hmm, I actually wonder if such a comment would make sense here. I can't
see why somebody interested in the RPC API would be looking at this
code. How about something like this:

  // Whether or not a series is active. This is serialized/deserialized
  // but for now there's no way to deactivate a series.