Zipkin exporter (#495)

* 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

example/zipkin/README.md

@@ -0,0 +1,16 @@
+# Zipkin Exporter Example
+
+Sends spans to zipkin collector.
+
+### Run collector
+
+```sh
+docker run -d -p 9411:9411 openzipkin/zipkin
+```
+
+### Run client
+
+```sh
+go build .
+./zipkin
+```