Closes #33

They now have a version and scanner fields

sbws will not be able to parse results without a version (and
won't parse results that don't have a version of 1).

Old results are not compatible moving forward because they
are missing this field

This isn't necessarily a bug. For example, we'll get a None if
there was no valid helper for the target relay.

- Allow one password per helper relay
- When measuring RelayFoo that happens to be a helper, don't use it
as a helper for that circuit