Add MD4,MD5; add warning about obsolete hash functions

[celeduc]

Resolves #71 by adding MD4 and MD5 to the list of supported hash functions.

Adds a note to the README cautioning about their use.

[celeduc]

@jbenet could you please look this over? I'm actually in need of this functionality.

[ghost]

I think these codes look good to me, what do you think @diasdavid @whyrusleeping

@celeduc could you also add these to the csv in multicodec? That one is the canonical table and multihash is just a subset. I dislike the duplication too, PRs very welcome :)

[daviddias]

Seems that there is no collision with https://github.com/multiformats/multicodec/blob/master/table.csv, as @lgierth said, we prefer to have the codecs first merged at the multicodec table then on the specs that dependent on it, so once we have that, we are golden to merge all of them PRs :)

[celeduc]

I submitted PR #41 to multicodec, where it's held with a comment by @Kubuxu

[celeduc]

Ready for merge!

[daviddias]

thank you @celeduc :)