Update Zeroconf class and add methods to remove listeners

[Rmannn]

The Zeroconf does not have any methods to easily remove listeners of DeviceEventEmitter.

This refractor adds life cycles calls and methods to allow listeners to be cleaned at start and stop (scan() & stop())

New methods:
- removeDeviceListener(string: event)
- removeAllDeviceListeners()

[balthazar]

Hi @Rmannn, thanks for your input and submission! Sorry for the delay.

I've decided not to accept your PR since it changed the initial behavior too much, by not registering listeners at instantiation, but instead in the scan and removing them on each stop.
Besides, you haven't checked if listeners were already set, thus resulting in multiple callbacks for the same event.

My implementation that landed in 0.5.0 allow you to removeDeviceListeners and addDeviceListeners at will. Update of docs will follow.

Do not hesitate if you have any question / remark!