Zipkin-specific observer support

[iamtakingiteasy]

Naive approach to adding support for zipkin-specific observers.

Merging two interfaces does not seem to be viable, either type-safety- or performance-wise.

closes #156

[iamtakingiteasy]

Fixed newly-introduced golangci-lint complaints regarding repeating string constants in test files in last force push.

There is still two complaints from upstream remaining, regarding parseTagsAsZipkinOptions cyclomatic complexity and benchmarkWithOps's numItems parameter always being 0.

Possibly it is worth disabling golangci-lint for *_test.go files and either splitting parseTagsAsZipkinOptions in few further functions, or raising gocyclo limit and/or using gocognit instead.

[iamtakingiteasy]

Cleaned up implementation a bit.

[iamtakingiteasy]

Added more test-cases.

[iamtakingiteasy]

Removed redundant FinisherWithDuration interface, as zipkin.Span already provides that method.
This in turns makes clear that FinishWithOptions() cannot return without actually finishing the span.

Added SetRemoteEndpoint in SetTag() as well as OnSetRemoteEndpoint() for ZipkinSpanObserver.