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.

Documentation/Architecture.md

@@ -95,6 +95,19 @@ While a few simple retries are done, there isn't yet a reliable
 message bus between Kubewatcher and the function.  Work for this is
 tracked in issue #64.
 
+Message Queue Trigger
+---------------------
+
+A message queue trigger binds a message queue topic to a function:
+events from that topic cause the function to be invoked with the
+message as the body of the request. The trigger may also contain a
+response topic: if specified, the function's output is sent to this
+response.
+
+Here's a diagram of the components:
+
+![Message queue trigger Diagram](https://user-images.githubusercontent.com/202578/27012344-9457cb24-4f00-11e7-8d6b-926ff01637b3.jpg)
+
 Environment Container
 ---------------------
 
@@ -113,7 +126,7 @@ volume shared between fetcher and this environment container.  Poolmgr
 then requests the container to load the function.
 
 Logger
------------
+------
 
 Logger helps to forward function logs to centralized db service for log
 persistence. Currently only influxdb is supported to store logs.