Fix "add event" event handling

[krnowak]

The Event field is nowhere set. Instead Message and Attributes
fields are filled with information from the event. So when handling
the "add event" event, read the Message and Attributes fields,
instead of Event, which is always nil. This avoids a crash.

I found this issue when testing my opentracing bridge with stdout exporter.

[krnowak]

For the record, the stack trace I was getting before the fix:

$ ./lesson01 Foo
2019/08/15 14-47-23.826314 start say-hello, a root span [ service=hello-world span_id=4d65822107fcfd52 trace_id=78629a0f5f3f164fd5104dc76695721d ]
2019/08/15 14-47-23.826429 modify attr [ service=hello-world hello-to=Foo span_id=4d65822107fcfd52 trace_id=78629a0f5f3f164fd5104dc76695721d ]
2019/08/15 14-47-23.826479 finish say-hello (164.892µs) [ service=hello-world span_id=4d65822107fcfd52 trace_id=78629a0f5f3f164fd5104dc76695721d ]
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x635568]

goroutine 1 [running]:
go.opentelemetry.io/experimental/streaming/exporter/reader/format.AppendEvent(0xc000098520, 0x3, 0xbf4d72f6f142beb8, 0xc9818, 0x8f3ca0, 0x5, 0x0, 0x0, 0x0, 0x0, ...)
	/home/kv/projects/lightstep/opentelemetry-go/opentracing-bridge/experimental/streaming/exporter/reader/format/format.go:76 +0x298
go.opentelemetry.io/experimental/streaming/exporter/reader/format.EventToString(...)
	/home/kv/projects/lightstep/opentelemetry-go/opentracing-bridge/experimental/streaming/exporter/reader/format/format.go:135
go.opentelemetry.io/experimental/streaming/exporter/stdout.(*stdoutLog).Read(0x9104d8, 0x3, 0xbf4d72f6f142beb8, 0xc9818, 0x8f3ca0, 0x5, 0x0, 0x0, 0x0, 0x0, ...)
	/home/kv/projects/lightstep/opentelemetry-go/opentracing-bridge/experimental/streaming/exporter/stdout/stdout.go:32 +0xab
go.opentelemetry.io/experimental/streaming/exporter/reader.(*readerObserver).orderedObserve(0xc0000fe000, 0x5, 0xbf4d72f6f142beb8, 0xc9818, 0x8f3ca0, 0x3, 0x0, 0x0, 0x0, 0x0, ...)
	/home/kv/projects/lightstep/opentelemetry-go/opentracing-bridge/experimental/streaming/exporter/reader/reader.go:297 +0x2c5
go.opentelemetry.io/experimental/streaming/exporter/reader.(*readerObserver).Observe(0xc0000fe000, 0x5, 0xbf4d72f6f142beb8, 0xc9818, 0x8f3ca0, 0x3, 0x0, 0x0, 0x0, 0x0, ...)
	/home/kv/projects/lightstep/opentelemetry-go/opentracing-bridge/experimental/streaming/exporter/reader/reader.go:121 +0x6c
go.opentelemetry.io/experimental/streaming/exporter/observer.Record(0x5, 0xbf4d72f6f142beb8, 0xc9818, 0x8f3ca0, 0x3, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
	/home/kv/projects/lightstep/opentelemetry-go/opentracing-bridge/experimental/streaming/exporter/observer/observer.go:141 +0x16f
go.opentelemetry.io/experimental/streaming/sdk.(*span).AddEvent(0xc000090ea0, 0x72a1e0, 0xc0000a2018, 0x727740, 0xc000098500)
	/home/kv/projects/lightstep/opentelemetry-go/opentracing-bridge/experimental/streaming/sdk/span.go:107 +0x14e
go.opentelemetry.io/experimental/bridge/opentracing.(*bridgeSpan).LogFields(0xc0000983e0, 0xc000118000, 0x2, 0x2)
	/home/kv/projects/lightstep/opentelemetry-go/opentracing-bridge/experimental/bridge/opentracing/lib.go:114 +0xe4
main.main()
	/home/kv/projects/lightstep/opentelemetry-go/opentracing-bridge/experimental/bridge/opentracing/tutorial-test/lesson01/hello.go:30 +0x36d

[tedsuo]

LGTM, Since this is experimental, going to merge quickly.