Pyramid-zipkin

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- Always generate ZipkinAttrs, even when a request isn't sampled.

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- Don't set parent_span_id on root span

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- Made generate_random_64bit_string always return str, not unicode

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- Fixed bug where headers were not 64-bit unsigned hex strings.
- Added ClientSpanContext, that lets users log arbitrary trees of
client spans.
- Deprecates "is_client=True" debug logging key in favor of a
non-None "service_name" key for indicating that a span logged
is a new client span.
- Batches up additional annotations in client before sending
to the collector.

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- Spans without a span ID will generate a new span ID by default.

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- Add ability to override "service_name" attribute when logging client
spans.