Ethernet shield support

First of all, there were some changes to the original Makefile to suit my linux, hope I didn't break anything for others.
I don't remember exactly what are those volatile in ethernet/Wiz5100.cpp for but I recall some troubles with SPI reads (and half of the uint16_t variable trashed sometimes, maybe after an interrupt). 
The code for DHCP is rather ugly, it worked, though.