Adding configs and pubsub implementations for GCP plus additional context endpoints and service types

[jprobinson]

This PR brings in some new types and tools for working with Google Cloud Platform:

• config/GCP: A helper config for working with Project IDs and contexts across development, App Engine, GCE and GKE
• config/PubSub: A helper config that is composed of a config.GCP and holds topics and subscription values.
• pubsub/GCPSubscriber: A pubsub implementation for subscribing to messages via GCP pubsub.
• pubsub/GCPPublisher: A pubsub implementation for publishing messages to GCP pubsub.
• server/JSONContextEndpoint: A new JSONEndpoint-like handler to work with context.Context
• server/JSONContextService: A new Service implementation composed of JSONContextEndpoints that can register with a SimpleServer.
• server/MixedContextService: A new Service implementation composed of JSONContextEndpoints and ContextHandlers that can register with a SimpleServer.

There is ONE major breaking change in this PR. The pubsub/Publisher interface now expects a context.Context value passed to the Publish methods:

+   // Publish will publish a message with context.
+   Publish(context.Context, string, proto.Message) error
+   // Publish will publish a raw byte array as a message with context.
+   PublishRaw(context.Context, string, []byte) error

The context object is currently being used by the GCP implementations, but is ignored by AWS and Kafka.

[coveralls]

Coverage decreased (-1.02%) to 44.947% when pulling d0b6b12 on gcp-pubsub into 53c5ab2 on master.

[coveralls]

Coverage decreased (-1.06%) to 44.91% when pulling 489f10d on gcp-pubsub into 53c5ab2 on master.

[coveralls]

Coverage decreased (-1.06%) to 44.91% when pulling 7d617b7 on gcp-pubsub into 53c5ab2 on master.