Specify the draft Verifiable Credentials v2.0 context.

[msporny]

This PR addresses PR #752 and issue #904 by specifying the Verifiable Credential v2 JSON-LD Context. The new context:

1. Removes all cryptography suites (until it becomes clear what the plan for future suites and the core context are) as requested by @OR13.
2. Removes the use of CURIEs and replaces them with full URLs as requested by @dlongley .
3. Adds the name and description VC properties as requested in PR #752 by @tplooker and @kdenhartog.

This is just an initial PR for a draft v2 context. It's expected that it will change as the WG progresses its discussions.