* Add zipkin exporter
The zipkin exporter implements the SpanBatcher interface. It follows
the current-at-the-time-of-writing document about conversion from
OpenTelemetry span data to Zipkin spans. Which means that endpoint
information is not yet filled.
* Fix typo in docs
* Add a zipkin example
This sends span information to a locally running zipkin collector.
Currently I have a problem getting the collector to show me the spans
after accepting them with HTTP 202. Not sure if this is because of
missing endpoint information.
* Make gitignore consistent
The fixed paths should be prefixed with a slash. The "relative" paths
mean that git will ignore all the files that end with the path.
* Add tests for zipkin exporter
