Introduced two new methods:

powerOff():
This will put the enc28j60 module into the sleep mode.

powerOn():
This will wake up the enc28j60 module back from sleep mode.

Usage:

void loop()
{
  Ethernet.maintain();
  [...]
  Ethernet.getNetwork().powerOff();
}
add Enc28J60.powerOff(), Enc28J60.powerOn() and Enc28J60.getrev()