Add ability to set explicit start & end time for span

[griels]

This is present in other implementations.
For example, Java has spanBuilder.withStartTimestamp.

It is crucial for situations where the start time is only known after an event has completed, for example when marshalling a span from a 'foreign' tracing system (this is my situation).

[Oberon00]

Spec reference: https://github.com/open-telemetry/oteps/blob/master/text/0002-remove-spandata.md

[griels]

Great, sounds like it's on the todo list :) Thanks.

[johananl]

Same for setting an explicit end time when calling finish() on a span. I guess we can handle both in this issue.