Ethernet added

[timmbogner]

I added ethernet yesterday. To use it, enable USE_ETHERNET. In my case I only need to add #define ETH_CLK_MODE ETH_CLOCK_GPIO16_OUT somewhere before setup(), and then bring my ethernet power pin (GPIO5) high in setup() to make it work.
@generalmanager I'm going to tag you when I commit my ethernet gateway example. Would you mind modifying my example to work with your Olimex boards?

[aviateur17]

@timmbogner, in fdrs_gateway_wifi.h line 93
WiFi.onEvent(WiFiEvent);
Should that be ETH.onEvent(ETHEvent);?
And then rename the callback on line 48 to ETHEvent?