[New] added Skein test for official test vectors

The Skein implementation is pretty broken. Many of the test vectors
fail:
- tree leaves are skipped
- if the blocksize is smaller than the digest: FAIL
- if the digest larger than the blocksize: FAIL - returned digest was
  same size as the blocksize. I added a temporary workaround in that i
  trim the digest to the requested size, as this seems to work.
- MAC seems to fail