added methods for serialization of pow objects

proofow.py

@@ -1,5 +1,6 @@
 from os import urandom
 from itertools import count
+from base64 import b64encode, b64decode
 from hashlib import sha256
 
 
@@ -28,6 +29,16 @@ class pow_base:
         hexresult = self._hexdigest(self._base + str(nonce).encode())
         return int(hexresult, 16) < self._target
 
+    def export(self):
+        return b64encode(str(self.difficulty).encode()
+                         + b':'
+                         + self.base).decode()
+
+    @classmethod
+    def load(cls, export):
+        difficulty, base = b64decode(export.encode()).split(b':', 1)
+        return cls(int(difficulty), base)
+
 
 class pow_hashlib_base(pow_base):
     def _hexdigest(self, data):