Adds support for message queue triggers based on the NATS-Streaming message queue.
This lets the user map a queue topic to a function, and optionally send the function's response into another queue topic.
The message queue trigger manager is designed to be message queue independent, so we should be able add support for more queues by adding implementations for the relatively simple mqtrigger.MessageQueue interface.
Test script that sets up fission on a cluster, runs tests and
uninstalls it.
Yet to come:
1. The actual tests
2. Builds (for fission-bundle, client, and envs)
3. Automation with github pull requests
