ENC28j60 Ethernet driver code

The ENC28j60 is a common Ethernet chip used by both the new OpenMote platform (http://www.openmote.com/), the do-it-yourself Thingsquare CC2538 setup (https://github.com/thingsquare/hardware/blob/master/ti/cc2538dk/assembly/README.md), and other upcoming platforms.

This pull request adds Thingsquare's ENC28j60 driver to the Contiki tree.