documenting  that on() handler accepts .trigger() args #472

See #472 for the complete story. Basically;

 1. Added `extraParameter` to the signature, and added further documentation as to how it works. It uses the `rest="true"` vararg attribute, which currently doesn't *show* anything, but support is being added in https://github.com/jquery/grunt-jquery-content/issues/56
 2. Added examples of using `on()` and `trigger()`.
 3. Various tidyings

... all separated into separate commits.
I personally don't like having demos with bad variable names (demos should show how real code should be written). I'm fine with whatever the consensus is.