Fix `Initialize Segment instance variables

Initialize stats and branch of the Segment class to None.

This should hopefully fix the issue outline in b-ryan#408:

```
AttributeError: 'Segment' object has no attribute 'stats'
```

powerline_shell/segments/hg.py

@@ -48,6 +48,9 @@ def build_stats():
 
 
 class Segment(ThreadedSegment):
+    def __init__(self):
+        self.stats, self.branch = None, None
+
     def run(self):
         self.stats, self.branch = build_stats()