Enhancements for the Identity registry

Identities code need some enhancement :

• Identity methods must not return Json data anymore. It should return Identity objects.
• Identity should be able to instanciate other Identities in requests methods
• Identity instanciating should do a "Certifiers-of", and if it fails, a lookup. Because the lookup is not necessary written in the blockchain, whereas certifiers-of is.
• We must add a "LOCAL_STATE" attribut describing : "COMPLETED/PARTIAL". When all data of an identity have been downloaded locally, it will be set as COMPLETED. In the meantime, it's PARTIAL.
• We must add a "BLOCKCHAIN_STATE" attribute describing "VALIDATED/BUFFERED". An identity is buffered until it is written in the blockchain, where it becomes "VALIDATED".
• Caching of identity should not save Json data anymore, only lists of pubkeys+uid corresponding to Identities.

@vtexier : I did most of work on the identities backend. I think the wot graph should not handle metadata anymore, but "Identity" objects return by the certifiers-of and certified-by objects. We should also handle "inner_data_changed" signal on certifiers and certified identity object to refresh the current wot graph.

These two methods now return a dict with 2 keys :

• "identity" : The certifier or certified Identity instance
• "cert_time" : An int, the median time of the certification