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.
This change adds a timer trigger API, client, and implementation. Users can trigger a function with a cron-compatible string.
This change also moves the publisher interface and webhook-based publisher implementation out of kubewatcher and into a separate `publisher` package.
Motivation: The output when an error occurs shows an integer error code,
which isn't very helpful. And in the case of an error when creating a
resource, you also get a redundant log message from the controller
client.
Modifications:
* the controller client no longer logs errors
* the generated String() allows for the enum name to be displayed in the
error output
* each errorCode enum member has a hand-generated description
* the format of the error message from the CLI client was modified to
display the error description with the error message
Related to #112
