Wrap `Span` for more scala idiomatic interface

[yannick-cw]

Span has stuff like addAnnotation, which I'd like to use, but returns unit, should we wrap it in sth. to have a more scala like api, returning the changed span? Same for the other Span methods @Sebruck ?

[Sebruck]

The only problem I see is that we might awake false assumptions on the client side. The fluent api then "feels" like it's immutable, but in fact it's not.

But I'd happy to play around a bit and see if we can come up with something useful!