Using effects and coeffects to keep re-frame handlers pure

We'll see some examples of using (and creating) effects (https://github.com/Day8/re-frame/blob/master/docs/Effects.md) and coeffects (https://github.com/Day8/re-frame/blob/master/docs/Coeffects.md) to keep re-frame (https://github.com/Day8/re-frame) handlers pure, and see how this makes its testing really easy.