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.
If no uid is provided, all versions of function files and function metadata will be deleted. If a uid is provided, only that specific version is deleted, and the "latest" version of the function moves back to the previous one.
Fixes#152
