Authorisations

[lovesh]

The DID spec leaves the implementation of authorisation to the DID Method specification.
The following authorizations are proposed:

1. ADD_KEY: Add new key to the DID doc. The new key can only have ADD_KEY authorization.
2. REM_KEY: Remove existing keys from the DID doc.
3. ALL: All possible authorizations. The key submitted during creation of DID doc has this authorisations. The key with this authorization can add keys with ADD_KEY, REM_KEY and ALL authorization

Any key can rotate itself, this is done using the id property