Pass QuorumSpec via WithQuorumSpec

[meling]

Pass QuorumSpec via WithQuorumSpec

[johningve]

One problem with passing QuorumSpec as an interface{} is that we won't get any compiler errors if the qspec object doesn't implement the QuorumSpec interface.

If the main idea is to get rid of the qspec argument in NewConfiguration when there are no quorum functions, then we could instead use templates to omit the qspec argument when it is not needed. That way we keep the compile-time type checking.

[meling]

I prefer not to generate a separate NewConfiguration method with a different signature for methods that don't use the QuorumSpec.

PR #122 implements a non-type safe approach. A draft generics idea is sketched here. However, we could do better with generics, e.g., I think we can do a single With[Type]() method.