Add DeviceData and eventHandlers.js

[liberty-rowland]

@mar-v-in Based on our conversation around nimiq-network/developer-reference#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.

[mar-v-in]

The description is misleading. The PoolAgent was already created at this time (because it is created before the first register arrives). There is also no reason why the PoolAgent instance is not a param on this function.

My suggestions would be
a) add PoolAgent param to beforeRegister
b) rename beforeRegister to onRegisterMessage
c) fix the description to state that it is fired before the message is processed by the PoolAgent
d) also rename onRegister to onRegistrationCompleted to make clear that it is fired after the registration.

[mar-v-in]

We use ES6 getters in our codebase instead.

[liberty-rowland]

@mar-v-in Addressed your concerns.

[mar-v-in]

Shouldn't this one be async as well?

[liberty-rowland]

I thought I'd leave it synchronous since I was catching with try/catch but I suppose this will still work as async.

[mar-v-in]

yes, exceptions do work across async/await

[mar-v-in]

missing await?

[liberty-rowland]

Good catch

[liberty-rowland]

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

[Zolmeister]

msg should definitely be a copy

[Zolmeister]

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