[all] pylint refinements

Improve code by bringing up the pylint score a bit.

see #23

bumblebee/store.py

@@ -6,13 +6,16 @@ this module
 """
 
 class Store(object):
+    """Interface for storing and retrieving simple values"""
     def __init__(self):
         self._data = {}
 
     def set(self, key, value):
+        """Set 'key' to 'value', overwriting 'key' if it exists already"""
         self._data[key] = value
 
     def get(self, key, default=None):
+        """Return the current value of 'key', or 'default' if 'key' is not set"""
         return self._data.get(key, default)
 
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