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
