Print revocation documents in QrCode format

Linked to https://github.com/ucoin-io/ucoin-python-api/issues/21

In UCPv0.1, the revocation document was just an indicator for nodes to remove an identity. But now, it goes deeper: the revocation is written in the blockchain saying:

This identity won't be usable anymore.

Also, a member sending such a document is immediatly excluded from the WoT.

Revocation documents have no expiry date, and can be printed on some paper to be written later on.