Add message queue trigger support (#218)

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.

INSTALL.md

@@ -200,3 +200,22 @@ To setup Fission-ui with fission in k8s is simple:
 Then open `http://node-ip:31319` to use Fission-ui.
 
 For more infomation, please check out [Fission-ui Readme](https://github.com/fission/fission-ui/blob/master/README.md).
+
+### Install NATS for message-queue based triggers (Optional)
+
+Fission supports message queue triggers that allow you to invoke
+functions based on events in a queue.  For now, NATS-Streaming is the
+only supported message queue.
+
+You can install NATS Streaming on your Kubernetes cluster with:
+
+```
+  $ kubectl create -f fission-nats.yaml
+```
+
+You can subscribe to a NATS Streaming queue with a command like this:
+(See `fission mqtrigger --help` for details)
+
+```
+  $ fission mqtrigger create --name myQueueTrigger --function processEvent --topic "myQueue.request" 
+```