Add standard (generic) JSON Schema for implementors to validate their verifiable credentials (see pending PR #17)

[dmitrizagidulin]

Consensus at the Barcelona f2f meeting on Mar 4 2019 was to add a JSON schema for the VC data model, to help implementors.
(Original issue: w3c/vc-data-model#128)

[burnburn]

@jonnycrunch volunteered to do this.

[jonnycrunch]

see: #17

[jonnycrunch]

WIP

[jonnycrunch]

updated, but could use some more testing.