Fix bugs with invalid credential ID and missing proof section when verifying credentials

For some reason the JSONLD processor library used by the Aries packages cannot properly create/verify signatures if the subject id is a simple string (e.g. not a DID/HTTP/Other address). It must either be an address with scheme:address or it must be missing (it's an optional field as defined by the VC spec).

Closes #18 (closed)

Closes #19 (closed)