Added support for ENC28J60 chips via ntruchsess' arduino_uip library

Added support for ENC28J60-based Ethernet Shields using [ntruchsess' arduino_uip library](https://github.com/ntruchsess/arduino_uip). I have only tested this with [e-Gizmo Ethernet Shield](http://www.e-gizmo.com/KIT/ethernet%20shield.html) since this is the only shield available locally in our country.