* add WithSpanKind option to span creation
* change SpanKind to string alias and add support for SpanKind on ot bridge
* fix tests
* fix import order
* fix nits
* Add Stackdriver Trace exporter for trace.
TODOs for future work is:
* to replace bundler.Bundler
* to add proper tests for the exporter
* to move the exporter to proper repository once it will be created.
* Change to use functions for the exporter initialization instead of
passing option struct directly.
This fix is aliging the same fix for Jaeger (#146, #161)
* Change Option struct to be function type
* Change the original Option struct to be private
* Add line comments to maxMessageEventsPerSpan to leave it for future implementation
* Fix unnessesary expressions specified by `make precommit`
Left errors by `make precommit` in experimental/bridge/opentracing.
* Ran make precommit
* Add new line at EOF
* WIP: Start implementing BatchSpanExporter interfaces
* Change to use RegisterSpanProcessor to register bsp
* Change function names to fit current implementation of sdk
* Removed google.golang.org/api/support/bundler and implement ssp and bsp
* Change spanProcessor as a member of Exporter.
* Fix option names used for BatchSpanProcessor initialization.
* Change Exporter.Shutdown just to unregister spanProcessor.
* Removed copyright statements of OpenCensus.
* Fix small typo and EOF new line
* Fix interfaces of ExportSpan/ExportSpans to meet SpanSyncer/SpanBatcher
* Change to follow context.Context passed in ExportSpan/ExportSpans
* Fix Stackdriver Exporter to hold sync.Once to lock when it is registered and
unregistered.
