added difficulty property for ease of use

proofow.py

@@ -8,7 +8,12 @@ class pow_base:
     def base(self):
         return self._base
 
+    @property
+    def difficulty(self):
+        return self._difficulty
+
     def __init__(self, difficulty, base=None):
+        self._difficulty = difficulty
         self._base = urandom(self._bits//8) if not base else base
         self._target = 2 ** (self._bits - difficulty)