v0.8.0: GCP event bus improvements, OpenTracing

Features

• GCP event bus improvements #270
• Tracing with OpenTracing #271

Fixes

• Add String() to aggregate, command and event type #270
• Make Matcher an interface #270
• Add observer groups for the observer middleware #270
• Add cancellable context to EventBus.AddHandler() #270
• Move min version context to the version repo #270
• Make waiting for projector entities optional #270
• Fix incorrect package names for cache Repo #271
• Rename trace.EventStore to record.EventStore #271
• Run TodoMVC example in Docker #271
• Use "eventhorizon:" prefix for error logging #271
• Implement Cause method on Errors that have a cause #272