Adding Skein hash to the mix

[DonaldTsang]

As a new add-on for Multihash (see multiformats/multihash#83
and multiformats/multihash#82)
I have added some JS code to the files to comply with the new proposed standard.

[DonaldTsang]

For future reference:
https://github.com/coiscir/jsdigest/blob/master/src/hash/skein.js

[daviddias]

@DonaldTsang thanks for adding this in! To make it official, the hash codes need to be added to the multicodec table first https://github.com/multiformats/multicodec

[DonaldTsang]

@diasdavid Done! <3