Signature model has signature_id attribute

[benoitbryon]

Just like signature_backend_id attribute identifies the signature on backend side, we need an identifier on local (Django) side. Because, just as in Django side we store external backend ID, in external backend we may store Django's id. Default primary key (integer id) is not enough. Something like an uuid would be welcome.

Another use case is URL construction: it is better if "signer return" and "callback" URL contain signature uuid:

• it is quick and easy to get the signature object matching the request. We can use a SingleObjectMixin whatever the backend.
• it is a bit harder for clients to "guess" a valid URL and try various POST/GET requests. They need a valid signature identifier.

UUID should be the default implementation, but of course this must be overridable.