fix setDrawsNotificationsAutomatically

Fixes #8

Previously setting this to false would prevent the server from being visible, as `updatedThisFrame` was only set to `false` in `draw`. If drawing was disabled this would never happen, and `updateServer` wouldn't be called.

It appears that `updatedThisFrame` was redundant, as it was only used to prevent calls to `updateServer` on the main thread when the server was set to threaded mode; `threadedUpdate` takes care of this well enough.

This update also moves the `ofAddListener` to `draw` to `setDrawsNotificationsAutomatically` so that it can be changed on the fly, enabling/disabling notification drawing before or after setup.