Simplify API by triggering listener immediately on subscribe for any value set

I removed the exception for undefined and null values, which now appears to me
artificial.

I also removed the Notes: section from the documentation of the function,
which I updated to express the behavior in a positive way rather than
describing limits of the behavior in the notes.
Make value parameter optional in publish(), with boolean value true as default

The change had already been described in the API section of README.md.