gh-34

We were previously building the json payload ourselves. Now, we off-load
that work onto a library which is made for this purpose.

This is just an initial shim that works. We probably need to ditch the
SlackPayload class entirely favoring a SlackMessage Factory that does
the work currently in the SlackPayload constructor.

Before this change, the Verbose setting wasn't able to be set to `True`
because it was the same value as the placeholder text.

This also cleans up a few warnings from the IDE

This simplifies what is essentially a functional operation into a static
function.