v3: W3C support

[Nebulis]

• agree on the document format :)
• digestCredential: write tests
• oa json-ld is currently hosted at https://github.com/Nebulis/tmp-jsonld/blob/master/OpenAttestation.v3.jsonld (for convenience) when ready, we must host it in this repository.
• salts is currently using b64 for compression. We must consider the following libs:
  • https://www.npmjs.com/package/messagepack
  • https://google.github.io/snappy/
• might consider a new salt format that reduces the data bloat, the original choice of uuid was out of convenience, for e.g just 256 random bytes to b64 or something (https://nodejs.org/api/crypto.html#crypto_crypto_randombytes_size_callback)
• is there a test for unicode key or values around somewhere? i think we had that in the original and it saved our butt
• audit
  • consensys diligence is a well known name in crypto community but i dont know how proven they are
  • i know ncc group is good outside of blockchain space
    audit library by nccgroup and audit blockchain by consensys diligence
• refactor (make it cleaner)

[yehjxraymond]

On the point for salts compression, I wonder if its necessary since the entire file could be compressed if its an issue.
I think we can work on non-uuid salts though

[Nebulis]

for the salt compression I'm fine, that was just a suggestion to make the output nicer =) no biggie from my side