Here's a patch that will implement RIPEMD-160 for nettle. It's code ported from libgcrypt. A reference to RIPEMD-160 is in the source. Also, this patch includes testcases and I followed what looked to me as nettle coding conventions.
There's no documentation update in this patch. I didn't look into how documentation was generated in nettle.
Also, this patch is against the nettle-2.2 release. I couldn't get the CVS snapshot of nettle building, particularly because of the integration of LSH.