Add DeviceData and eventHandlers.js

[liberty-rowland]

@mar-v-in Based on our conversation around https://github.com/nimiq-network/developer-reference/pull/24

This PR does a few things:

1. Adds knowledge of deviceData to PoolAgent
2. Adds public read-only properties to PoolAgent, since we will now be exposing it to pool owners. Properties are: deviceId, deviceLabel, mode, isRegistered
3. Added eventHandlers.sample.js and support for eventHandlers.js which allows pool owners to easily add handlers for events. I started with onRegister and beforeRegister but this can be expanded on.

[liberty-rowland]

@mar-v-in Addressed your concerns.

[liberty-rowland]

Thanks @mar-v-in, addressed second round of comments.

[Zolmeister]

This feels like it should be middleware.js (instead of having events trigger side-effects)