Support multiple network interfaces

[per1234]

EtherEvent currently is hard coded for Ethernet only but it should be possible to expand it to support any communication library that uses the Client class by passing the client object to EtherEvent.

Partial list of Target Libraries: https://github.com/ekstrand/ESP8266wifi - ESP8266 as serial module with AT firmware https://github.com/esp8266/Arduino/tree/master/libraries/ESP8266WiFi - ESP8266 standalone Arduino stock WiFi library https://www.arduino.cc/en/Reference/WiFi101 https://github.com/ntruchsess/arduino_uip - I've tried using this in the past and gave up because the output is not the same as the Ethernet library so EtherEvent would need to be made more flexible to support this.

References:

• https://github.com/knolleary/pubsubclient
• https://github.com/adafruit/Adafruit_MQTT_Library
• https://github.com/exosite-garage

Proof of concept test library: NetTest4.zip

To Do:

• Do a release before this change
• How to handle remoteIP()?
• EtherEventQueue compatibility
• Update examples
• Update documentation
• Major version bump