Hello, alvarojusten!
This is great idea to make ENC28J60 - W5100 compatible library!
I spent a lot of time with "official" ENC28J60 lib, yes it works, but any changes or any code writing are kinda hell.
But accidentally, I found another realization of the library:
http://jeelabs.net/projects/cafe/repository/entry/EtherCard
EtherCard based on official ENC lib, but there are a lot rewritten code, DNS realization, cool http server, new tcp-ip stack & etc.
For compatibility with Arduino you must change
#define ENC28J60_CONTROL_CS 8
to
#define ENC28J60_CONTROL_CS 10
at enc28j60.cpp
at least for me it worked.
This lib works not 100% stable, but it pretty simple than original lib.
I hope it can help you or others.
Please continue to develop your library, we all really need for it
Regars.