Implementation for Microchip ENC28J60 Ethernet controller

chiva:
Knowing that D2 isn't used in the libraries, that makes it pretty easy to add compatibility with Mega boards, and in fact I have already sent the code to alvarojusten and should be added soon to the libraries.

I accepted your pull request. Now supporting Mega! Thanks.

chiva:
Also, Sure electronics's ENC28J60 based ethernet board is also compatible with the libraries as expected, but couldn't find any information about that before buying it.

I'll contact them later.