Refactors signing/verifiying functionality to be part of the Cryptographer

- All encryption/decryption and signing/verifying calls are performed by the cryptographer now.

- The current signature format is temporal. We should define something not base on json.

- Some Cryptographer tests are still missing.

- The cli tests should be modified to fit this too.

pisa/api.py

@@ -1,7 +1,6 @@
 import os
 import json
 from flask import Flask, request, abort, jsonify
-from binascii import hexlify
 
 from pisa import HOST, PORT, logging
 from pisa.logger import Logger
@@ -40,7 +39,7 @@ def add_appointment():
 
         if appointment_added:
             rcode = HTTP_OK
-            response = {"locator": appointment.locator, "signature": hexlify(signature).decode("utf-8")}
+            response = {"locator": appointment.locator, "signature": signature}
         else:
             rcode = HTTP_SERVICE_UNAVAILABLE
             error = "appointment rejected"