fix: migration to new multiformats

[rvagg]

BREAKING CHANGE!

now import { encode, decode } from '@ipld/dag-json'

update README
add LICENSE files

Closes #27

[Gozala]

Looks great thanks for updating it @rvagg. My only concern is that this does not use provided codec function and just implements interface, so any changes made would need to be coordinated.

[Gozala]

I would suggest following instead:

import { codec } from 'multiformats/codecs/codec'
export default codec({ name, code, encode, decode })

Because it will:

1. Create a codec that is implements both encoder and decoder ends
2. Expose encoder and decoder ends separately.
3. If we do end up adding some kind of contaminators, dag-json would automatically get that as well.

[Gozala]

Just got to the dag-pb PR where some of this was discussed. I posted a bit more justification to my position there ipld/js-dag-pb#3 (comment)