add Schtick.prototype.ref and Schtick.prototype.unref

Since a Schtick instance wraps a timer, it can sometimes be convenient to have access to `ref()` and `unref()`. 

Possible use case is having some sort of scheduled task running in a web server process, but you don't want the  task to prevent the shutting down of the process.

I know you can just call `stop()` from the shutdown process, but calling `unref()` on the task after creation can be helpful in other scenarios, such as managing a scheduled task inside a library abstraction.
Pretty sure this is supposed to be `unref()`. Fix, and add documentation in the README, and I will accept this PR.