Ensure created proof option is optional

[dlongley]

The current proof configuration algorithm seems to make created required instead of optional, this should be fixed.

[msporny]

The created property is now optional:

https://w3c.github.io/vc-data-integrity/#proofs

The algorithms still need to be updated to ensure that created is optional (it shouldn't throw an error if it isn't specified).

[Wind4Greg]

@msporny, whoops, the EdDSA spec is using the "old language" in the DataIntegrityProof section. I'll update it by the end of the day PDT to use language like that in the ECDSA spec.

[msporny]

PR #86 was raised and merged to address this issue. Closing.